# Simple math formulas, please! **URL:** \ **Category:** Factual Questions\ **Created:** [September 22, 2007, 5:18pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116 "2007-09-22T17:18:07Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [September 22, 2007, 5:18pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/1 "2007-09-22T17:18:07Z")
What is the formula for X is 28.9% of Y? (where X is a KNOWN number and the number I need is Y) ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 22, 2007, 5:20pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/2 "2007-09-22T17:20:16Z")
The way I learned it, ‘of’ can be thought of as ‘times’. So X = 0.289 \* Y Edit: Sorry, you’re looking for Y. Y = X / 0.289 ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [September 22, 2007, 5:24pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/3 "2007-09-22T17:24:13Z")
never mind ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [September 22, 2007, 5:27pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/4 "2007-09-22T17:27:39Z")
What I wanted to write: When I need to figure out what formula to use, I plug in simple numbers and see what would work. Lets make it: X is 50% of Y Lets plug in simple numbers for X and Y that we know make a correct equation: 5 is 50% of 10 What simple equation can we make with 5 and 50 to get an answer of 10? ---
**Author:** ![lynne-42](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lynne-42/32/3138_2.png) [@lynne-42](https://boards.straightdope.com/u/lynne-42)\ **Post date:** [September 22, 2007, 10:52pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/5 "2007-09-22T22:52:00Z")
‘percent’ means ‘out of 100’. Any percentage is found by dividing by 100. 50 percent = 50 out of 100 or 50/100. What you have written in words is X = 28.9% of Y (of = multiplied by) which is X = 28.9 / 100 x Y which means X x 100 = 28.9 x Y which means X x 100 / 28.9 = Y So Y = X x 100 / 28.9 (the same result as **Johnny** said) ---
**Author:** ![iftheresaway](https://avatars.discourse-cdn.com/v4/letter/i/ecae2f/32.png) [@iftheresaway](https://boards.straightdope.com/u/iftheresaway)\ **Post date:** [September 22, 2007, 11:36pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/6 "2007-09-22T23:36:18Z")
I don’t know if this will help you, but the way I always remember it is with the formula “ **is** over **of** equals **percent** over a **hundred** ”, which would look like is/of = %/100 So if X **is** 28.9% **of** Y, the problem would read: X/Y = 28.9/100 Solve for Y by cross-multiplying: 100X = 28.9Y Not really different from what everyone else has said, just that the mnemonic is easier for me to remember (is that considered a mnemonic? am I spelling mnemonic right?) HTH. ---
**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\ **Post date:** [September 22, 2007, 11:49pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/7 "2007-09-22T23:49:58Z")
[QUOTE=x-ray vision] What simple equation can we make with 5 and 50 to get an answer of 10? [/QUOTE] Is this a trick question? ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [September 22, 2007, 11:55pm UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/8 "2007-09-22T23:55:37Z")
[QUOTE=x-ray vision] What I wanted to write: When I need to figure out what formula to use, I plug in simple numbers and see what would work. Lets make it: X is 50% of Y Lets plug in simple numbers for X and Y that we know make a correct equation: 5 is 50% of 10 What simple equation can we make with 5 and 50 to get an answer of 10? [/QUOTE] Unfortunate example. Anyone facing that will say “Well, 50/5 = 10”. Which is entirely misleading. (It’s not as though “4 is 20% of Y” implies that Y = 20/4 = 5.) The original equation you really want is 5\*(100/50) = 10. But who’s gonna come up with that when posed with “What simple equation can we make…?”? Not that your method for figuring problems out is devoid of merit. But ideally, it steers one towards (and enables one in) doing some conceptual analysis of the problem at hand, rather than, as is a danger, leading to blind casting about for something that “works”. ---
**Author:** ![gyushi](https://avatars.discourse-cdn.com/v4/letter/g/b4bc9f/32.png) [@gyushi](https://boards.straightdope.com/u/gyushi)\ **Post date:** [September 23, 2007, 2:30am UTC](https://boards.straightdope.com/t/simple-math-formulas-please/420116/9 "2007-09-23T02:30:34Z")
The actual best way to think of it is to remember that the symbol “%” is simply shorthand for the fraction “1/100”, or the number “0.01”. So 28.9% is equal to (28.9)(1/100) or (28.9)(0.01). So if X = 28.9% of Y, X = (28.9)(0.01)(Y) X = (0.289)(Y) Y = X / 0.289