# maths question how to reverse a percentage calc. **URL:** \ **Category:** Factual Questions\ **Created:** [March 6, 2007, 7:44pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842 "2007-03-06T19:44:46Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Lobsang](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobsang/32/4067_2.png) [@Lobsang](https://boards.straightdope.com/u/Lobsang)\ **Post date:** [March 6, 2007, 7:44pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/1 "2007-03-06T19:44:46Z")
Given the sum x - (x \* 0.25) = y how do I work out what x was if the only known number is y ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [March 6, 2007, 7:48pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/2 "2007-03-06T19:48:54Z")
If you multiply both sides of the equation by 4, you get x=4y. ---
**Author:** ![brewha](https://avatars.discourse-cdn.com/v4/letter/b/91b2a8/32.png) [@brewha](https://boards.straightdope.com/u/brewha)\ **Post date:** [March 6, 2007, 7:53pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/3 "2007-03-06T19:53:10Z")
No, you get 4x-x=4y 3x=4y x=(4/3)y ---
**Author:** ![Punoqllads](https://avatars.discourse-cdn.com/v4/letter/p/d2c977/32.png) [@Punoqllads](https://boards.straightdope.com/u/Punoqllads)\ **Post date:** [March 6, 2007, 7:53pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/4 "2007-03-06T19:53:14Z")
I don’t. But perhaps reformatting the problem will make the answer more apparent: 25% is 1/4. So: x - 0.25 \* x = y turns into x - 1/4 \* x = y ---
**Author:** ![Rysto](https://avatars.discourse-cdn.com/v4/letter/r/ecccb3/32.png) [@Rysto](https://boards.straightdope.com/u/Rysto)\ **Post date:** [March 6, 2007, 7:53pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/5 "2007-03-06T19:53:45Z")
No you don’t. You get: 3x = 4y x = (4/3)y ---
**Author:** ![SiXSwordS](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@SiXSwordS](https://boards.straightdope.com/u/SiXSwordS)\ **Post date:** [March 6, 2007, 8:07pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/6 "2007-03-06T20:07:15Z")
If the _only_ known number is Y, you can’t do anything. Y is what percent of what? Doesn’t go anywhere. If you call the percentage P ( .25 in you equation from the OP) and that is the number you’re after then you have to know x. (x-y)/x = P (y ≠ 25 percent of x) But it is simpler to use y/x = P (What percentage of X is Y) Instead of x - (x\*25) = y Use x \*.75 = y (y = 75% of x) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [March 6, 2007, 8:20pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/7 "2007-03-06T20:20:16Z")
[QUOTE=Lobsang] Given the sum x - (x \* 0.25) = y how do I work out what x was if the only known number is y [/QUOTE] x – (x \* 0.25) would be 0.75\*x. (You can either use your powers of algebra to combine like terms: 1x – 0.25x = 0.75x, or just think that if you subtract off 25% of whatever x is, you have 75% of it left.) Then divide both sides by 0.75: x = y / 0.75 (which agrees with what others have said, since dividing by 0.75 = dividing by 3/4 = multiplying by 4/3) ---
**Author:** ![Lobsang](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobsang/32/4067_2.png) [@Lobsang](https://boards.straightdope.com/u/Lobsang)\ **Post date:** [March 6, 2007, 8:39pm UTC](https://boards.straightdope.com/t/maths-question-how-to-reverse-a-percentage-calc/394842/8 "2007-03-06T20:39:00Z")
[QUOTE=Rysto] No you don’t. You get: 3x = 4y x = (4/3)y [/QUOTE] Thanks all. Turns out I’d guessed at the right modification before asking. x = y \* 1.333333333333 (which is the same as x= (4/3)y) It wasn’t giving accurate figures (out by up to .1) but that’s because the thing I was using to check my figures was rounding down every time)