Photograph via snooOG

And they said math has no real world applications

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  1. Proper title tags are required for all posts or it will be removed! The tags accepted are at the end of the sidebar down there.

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  3. Give credit where credit is due—include the user who made the calculations in any post you submit: ex. "[RDTM] /u/FragTheWhale calculates...".

  4. For easy and quick math results (ex. How many feet are in a mile?) use Wolfram|Alpha™. Posts containing "simple math" will be removed, as well as requests whose answers are easily searchable online, and any other post at the moderators' discretion. For more abstract math, try /r/math or /r/learnmath.

  5. Posts which seem to be asking for help on homework will be removed. Try /r/homeworkhelp or /r/cheatatmathhomework instead.

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  1. This subreddit has a zero-tolerance policy for hateful or unnecessary language. There is no warning for hate speech of any kind. Rule breaking posts/comments will have a temporary ban that will serve as a first offense "warning". Repeat or egregious offenders will be banned permanently. Reddit's site-wide rules still apply here. Also, don't be a dick.

  2. Do not submit top-level comments in [Request] posts that are not an attempt at an answer or a request for clarification. Jokes or off-topic top-level comments will be removed.

  3. When answering [Request] posts, please cite reputable sources to defend any evidence that cannot be proven using your own math.

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We require title tags. Here are the tags we accept and what they are for. Please keep them at the start of your post.

  • [Request] - Asking the /r/theydidthemath community to do the math for you!

  • [Self] - You did the math and want to share it!

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1,491,297 Subscribers


[Request] What is the ratio of innocent civilians killed by American police to civilian lives saved by them?

1 Comment
21:44 UTC


[Request] What are the odds for a teacher to never be chosen by a group of students ?

Hello everyone,

I'm struggling with the calculations of probabilites. I set up a backtracking algorithm that allows me to assign students to teachers. I'm trying to figure out the probabilities that a teacher is never assigned.

The setup is simple : I have a number of N students and N teachers. Each students has X wishes. Each student can only choose a teacher once, but a teacher can obviously be chosen by many students.
Assuming N = 20, I'm trying to calculate the probability that one teacher is never chosen (which would mean that the backtracking would fail to find a solution). Also, starting from X = 3 wishes, I'm trying to calculate the same probabilities for X = 4, 5, 6... until 20, so that I can find an amount of wishes from which I'd be safe to say the algorithm will (almost) always succeed, and convince anyone that increasing the amount of wishes is a great solution to prevent the algorithm to fail.

Can someone help me with that ? I'd be really grateful ! Thank you !

21:01 UTC


[Request] How Many Km Of RNA Is Needed To Vaccinate The Entire World?

19:47 UTC


[Request] If everything invented by Euler was named after Euler, How many Eulers things would there be?

19:37 UTC


[Request] How many humans on bicycles with generators would it take for me to plug a toaster into them and kill myself in a bath?

19:31 UTC


[Request] Please help me calculate trade spend

I am interviewing for a job and they sent me home with a project due on Monday. Please help explain this to me where it makes sense.

The only information I have is below.

White tag retail= 1.49 List cost = 1.05

Week one retail 1.25 Week one cost 1.02 Total units sold 249,152

Week two retail 1.00 Week two cost .85 Total units sold 458,952

And so on for a few weeks.

I thought it was the difference between cost and retail but that is the Margin.

Any help is appreciated!

1 Comment
19:29 UTC


[request] expected rolls to get all numbers twice

i have the answer, but it is solved by a big markov chain, which is not practical.

is this the only way? any short solutions?

preferable using recursion

if so, please do the math


1 Comment
19:27 UTC


[Request] How to suck the best?

I am trying to build my own version of a dust separator by modifying a wall mounted shop vac that I already have with the idea in this video and I am not sure what the best dimensions and locations are. I have a super dinky workshop; I really need to make this work over a commercially available system as I just don't have the room. Airflow stuff is probably a tricky beast, and I don't have a bunch of money to try different scenarios.

(I am not looking for exact dimensions, but more of some guidance on each of the major parameters. I would definitely take some exact dimensions, but not expecting it)

Green: should the inlet be higher or lower?

Yellow: should the vortex section be larger or smaller?

Pink: Should the vortex center tube inlet opening be smaller or larger (in both (either) directions tall and deep). this inlet tube would be 2.5" PVC.

Purple: Should the vortex center tube inlet be center, up, or down? should it be equal to, or offset in one direction or another from the inlet (Green)

Red: How much of an opening should the Baffle have from the side wall? (drawing left it out, but the inside diameter of this wall is 275mm)

Teal: what should the clearances be between the outlet of the vortex tube and the bottom of the filter?


17:41 UTC


[Request] Home Alone House Tourist attraction

If someone were to renovate the home alone house back to how it looked in the movie and dressed it up for guests to either get to tour the house or rent it out (like an air bnb) how much would you have to make a night to make profit?

Taxes alone are $50,000/yr


1 Comment
17:27 UTC


[Request] Math ain’t Mathing

If false, what would be the number of times you’d have to fold the paper?

16:31 UTC


[Off-Site]I really have no clue how to start solving this

15:47 UTC


[Meta] Can we ban posts about wealth?

It feels like every second post on this sub is someone asking, "Is this correct?" with a picture of some Twitter comment that says "If Musk/Gates/Bezos/etc shared just half of their net worth, all of America could eat free hamburgers for three years. How is this fair?" and the answer is almost always the same thing: "The numbers are correct, but that's not how money works." In other words, the discussion ends up being less about maths and more about basic financial and economic topics.

I'd like to know if anyone else also feels this way. Are the rest of you also annoyed with these posts or is it just me?

15:39 UTC


[Request] Is this truth and accurate?

As a regular job, five days at 40 hours per week or 160 hours per month compared to three begging scenarios of 6, 8 and 4 hours per day for 25 days per month.

So I read this article here that compares begging vs basic salary in Athens, Greece and I would like to ask if these numbers are truth and accurate. If it happens to be truth, this means begging is a viable option that earns more than basic salary in Athens?

14:48 UTC


[Self] Tradesman asking a sort of physics-related question about circular motion

Hi everyone, this is my first post on this subreddit

I'm a tradesman. I've been in the trades since 2011. I've worked as an Ironworker and as a Scaffolder; I'm a journeyman in both trades.

Anyways, this is a thought experiment I've been playing on the guys for the last many years:

Let's say you have a chop saw or an angle grinder, with fresh blades and worn-down blades. The fresh blade is brand new and obviously a larger diameter. The worn-down blade is smaller.

If you're running the saw with a constant engine RPM, does the outer edge of the fresh blade spin faster than the edge of the worn-down blade?

My answer is: yes, due to the outer edge of the fresh blade having to move a larger circumference. It literally travels a longer distance at the same RPM as the worn-down blade so it must be moving faster.

Surprisingly, people get my question wrong a lot. I've even asked civil engineers and most of them think the blades spin at the same speed. When I explain my reasoning it clicks in peoples heads and then they agree with me.

I just want to ask the smarter people of reddit . Am I right?

I think I'm right, but I've only really used reasoning I think and some basic mathematics principles to provide an answer. Is this an elegant enough proof?

Anyways, in real world application a fresh blade literally cuts faster than a worn-down blade, so using that thought it stands to reason the blade must be moving faster to cut faster.

I don't know. Maybe this is like a stoner-tradesman question.

I look forward to replies, thanks for the help.

14:41 UTC


How much food colouring is needed to significantly change the colour of the ocean? [Request]

Just curious

12:47 UTC


Sort of [self], Calculating the odds of two people getting the same Spinda in gen 3 pokemon via birthday paradox


You may have heard of the birthday paradox. If you haven't, it's basically the idea that with 23 people in a room, you have about even odds of two of them sharing a birthday. Now this sounds unintuitive, because that number seems quite small for a 1/365 chance happening twice. But this is basically because we forget to check between every pair of people, so there are actually 253 pairs to check, not just 23 ((23 x 22)/2 = 253).

Now, I was watching a video of a guy who broke himself trying to play Doom on the face of a Spinda (long story but fun video, I'll edit and link this to the video if I can, otherwise I'll try to put it in the comments. Edit: link to video). For those of you who don't know, Spinda is a bear-like pokemon introduced in Pokemon Ruby and Sapphire in 2002. It's unique in that it's face uses a 32-bit code to generate the positions of four spots on its face, which means there is 4,294,967,296 different possible versions of this pokemon (it also means you can make animations on its face by spoofing different 32-bit codes, hence Doom guy).

This got me thinking, with that many unique Spinda, what were the chances that two people who organically came across and caught this pokemon managed to get the same one? And hence me going down my own little rabbit hole.

From Wikipedia, the birthday paradox can be calculated via the following:

From a permutations perspective, let the event A be the probability of finding a group of 23 people without any repeated birthdays. Where the event B is the probability of finding a group of 23 people with at least two people sharing same birthday, P(B) = 1 − P(A).

P(A) is the ratio of the total number of birthdays (Vnr) without repetition or order matters divided by the total number of birthdays with repetition or order matters, (Vt). Therefore Vnr and Vt are permutations.

Vnr = n!/(n - k)! = 365!/(365 - 23)!

Vt = n^k = 365^23

P(A) = Vnr/Vt ~= 0.492703

P(B) = 1 - P(A) ~= 0.507

I thought that we could probably do the same by just subbing in possible combinations of Spinda and units of pokemon Sapphire Ruby and Emerald sold.

From a brief google, units sold is equal to 23,280,000. Like I mentioned before, Spinda has a possible 4,294,967,296 different forms.

So the expression should be:

Vnr = n!/(n - k)! = 4,294,967,296!/(4,294,967,296 - 23,280,000)!

Vt = n^k = 4,294,967,296^23,280,000

Now these are both insane incalculable numbers, and while I have no clue what I'm doing in general when it comes to maths, I do know that there's no way even the birthday paradox was calculated this way at first, seeing as factorial numbers are only really calculated to about 170! on most calculators due to limitations of current computing. So I read further.

Let the 23 people be numbered 1 to 23. The event that all 23 people have different birthdays is the same as the event that person 2 does not have the same birthday as person 1, and that person 3 does not have the same birthday as either person 1 or person 2, and so on, and finally that person 23 does not have the same birthday as any of persons 1 through 22. Let these events be called Event 2, Event 3, and so on. Event 1 is the event of person 1 having a birthday, which occurs with probability 1. This conjunction of events may be computed using conditional probability: the probability of Event 2 is 364/365 as person 2 may have any birthday other than the birthday of person 1. Similarly, the probability of Event 3 given that Event 2 occurred is 363/365 as person 3 may have any of the birthdays not already taken by persons 1 and 2. This continues until finally the probability of Event 23 given that all preceding events occurred is 343/365. Finally, the principle of conditional probability implies that P(A′) is equal to the product of these individual probabilities:

P(A') =365/365 x 364/365 x 363/365 ... x 343/365

which can be collected to arrive at

P(A') = (1/365)^23 x (365 x 364 x 363 ... x 343)

Evaluating equation (2) gives P(A′) ≈ 0.492703

Therefore, P(B) ≈ 1 − 0.492703 = 0.507297 (50.7297%).

At this point my head has started hurting and all the numbers are laughing at me as my eyes glaze over, but I try to push forward. I should be able to sub in Spinda combinations for days of the year, and units sold for people in the room.

4,294,967,296 - 23,280,000 = 4,271,687,296

P(A') = (1/4,294,967,296)^23,280,000 x (4,294,967,296 x ... x 4,271,687,296)

This is also an insane equation and I am starting to lose my mind, but at the very least we now know that the odds of two kids having the same Spinda is (1 - (1/4,294,967,296)^23,280,000 x (4,294,967,296 x ... x 4,271,687,296)), which means absolutely nothing to anyone and I have no way of calculating.

At this point I'm losing hope of ever knowing this but as I scroll down the Wikipedia page, I see a sub heading.


Finally, maybe there's some easy to use formula here that I can quickly find an answer with! I scroll through the section.

As soon as I get to the words "ceiling function" my brain collapses. What the hell is Asymptotic Density? What the hell is a hash function!? I trawl through trying to understand a single word, and give myself a headache as I start following deeper into the rabbit hole until reading about Stirling Numbers and Key Derivation Functions. It's at this point that I have fully given up on understanding anything ever again. But at least now we know something. Maybe.

So I thought I'd just put this here and maybe my brief foray into this madness will entertain you lovely people, either as a mildly interesting calculation or as another story of Icarus flying too close to the sun in his hubris. If any of you eggheads out there can do anything with these insane scribblings then please let me know. I need a damn drink.

12:42 UTC


[Request] How many reddit posts have I seen?

On average, I scroll through the "All" page about 1.5 to 2 hours a day. I have been a user since the last 10 years.

1 Comment
11:52 UTC


[Request] Calculate the height of a cliff jump

I was cliff jumping today and curious on the height of the jump. I figured using the time it takes to reach the water from the moment I leave the ledge that it could be calculated to find the height. Wind is essentially not a factor here today. My jump outward was around 6 feet or so although it appears more in the video but that’s only because the ledge slants inward and makes it falsely appear that I jumped further

11:37 UTC


[request] "in a room full of 1000 people, you would be smarter than 944 of them" what's my IQ?

Got scammed in a "free" IQ website that I needed to pay for my actual IQ, and could only see the phrase in the title for free

So what's my IQ?

11:26 UTC


[Request] What could the surface area of the earth tend towards with an advanced civilisation?

Not the spherical surface 510 millions square kilometre but the fractal surface area, assuming just for this calculation that we are very arbitrarily limiting ourselves to anything above 100 micrometers (hard to manipulate by most human below that). Activities such as life, technology, urbanisation, mining, deforestation are changing this surface so what ultimate fractal dimension could a technological civilisation tends to (if it does)?

11:25 UTC


[request] help me solve this please. Warning I’m not a student just an enthusiastic nerd like adult

10:18 UTC


How many calories are in the people in the image? [Request]

08:53 UTC


[Request] What are some things that are more likely to happen to a person than becoming a billionaire?

Assuming 120,000,000,000 people have ever lived and less 5,000 billionaires have ever existed, what are some things more likely to happen to someone. Back to back Powerball wins? Surviving the Bubonic plague and Syphilis after getting struck by lightning?

08:46 UTC


[Request] - How many combinations of 9 ingredients are possible. Using all 9 at once is not required.

08:18 UTC


[Request] Is this real and if yes why?

08:04 UTC


[REQUEST] can someone please help me solve this?

Wouldn’t it be 40 x 38 X 15 which he is 22800 ft? I’d hate to double it because then the result would be 45600 thst doesn’t seem right :s

07:47 UTC


[REQUEST] please help me solve this

I’m not sure if the formula would be WxLxH or if it’s S squaredxH

06:27 UTC


[request] If my pint of beer is poured 1cm short and it cost me £5 how much did I lose?

I know a pint is 568ml so if easier we could say the pint cost £5.68.

I know the older style Nonic pint glass with a bulge at the side is 8.1cm diameter at the top and 6.5cm at the base x 14.3cm high.

( Inspired by this: https://www.bbc.co.uk/news/articles/c3ggn9v1z7jo )

06:15 UTC

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