# Math Question - There must be some way ... **URL:** \ **Category:** Factual Questions\ **Created:** [October 23, 2013, 2:07am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997 "2013-10-23T02:07:37Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![urban1a](https://avatars.discourse-cdn.com/v4/letter/u/e47c2d/32.png) [@urban1a](https://boards.straightdope.com/u/urban1a)\ **Post date:** [October 23, 2013, 2:07am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/1 "2013-10-23T02:07:37Z")
When I buy medical supplies (DME), although this question applies to other purchases also, I only pay a percentage of the full price (20% if it makes any difference). I believe that since I know what I paid, there should be a way of figuring out what the 100% cost is. I’m very much afraid that this involves the use of the dreaded mathematical esoteric of algebra which I was never able to get through my head. But couldn’t it be more simple? The equation should look something like this: fullprice = ((amountipaid)\*( … )) I know that I need to multiply because I need to increase the amountipaid by some amount. Please suggest an equation to replace the ellipsis so I can sleep happy tonight. Oh, yeah, no hurry. Bob ---
**Author:** ![urban1a](https://avatars.discourse-cdn.com/v4/letter/u/e47c2d/32.png) [@urban1a](https://boards.straightdope.com/u/urban1a)\ **Post date:** [October 23, 2013, 2:08am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/2 "2013-10-23T02:08:39Z")
WOW 731 posts in 11 years. First time I’ve noticed that. Bob ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [October 23, 2013, 2:12am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/3 "2013-10-23T02:12:05Z")
> [@urban1a](#): > > When I buy medical supplies (DME), although this question applies to other purchases also, I only pay a percentage of the full price (20% if it makes any difference). I believe that since I know what I paid, there should be a way of figuring out what the 100% cost is. I’m very much afraid that this involves the use of the dreaded mathematical esoteric of algebra which I was never able to get through my head. But couldn’t it be more simple? > > The equation should look something like this: > > fullprice = ((amountipaid)\*( … )) > > I know that I need to multiply because I need to increase the amountipaid by some amount. > > Please suggest an equation to replace the ellipsis so I can sleep happy tonight. > > Oh, yeah, no hurry. > > Bob If you pay 20% of the full price, then the full price is 5 times what you pay. More generally, the full price is 100/x times the price you pay, if x is the % you pay. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 23, 2013, 2:14am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/4 "2013-10-23T02:14:41Z")
You pay (0.2)(fullprice) = (your cost) so the fullprice would yourcost divided by 0.2 ---
**Author:** ![Martin\_Hyde](https://avatars.discourse-cdn.com/v4/letter/m/47e85d/32.png) [@Martin\_Hyde](https://boards.straightdope.com/u/Martin_Hyde)\ **Post date:** [October 23, 2013, 2:23am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/5 "2013-10-23T02:23:05Z")
This is not Algebra, just basic division. Where x = the full price, c = your cost, and p = your cost’s percentage of full price: x = c / p So if you are paying $100, and you know that you are paying 20% of the total cost: x = 100 / .20 x = 500 A way to think about this is recognize a percentage is just some number out of 100. So if you are paying 20 percent, you are paying 20/100 of the cost. That’s the same as 1/5. So if you know your cost is 100, which is equivalent to 1/5 of the total cost, then it should intrinsically make sense that to find out the total cost you just need to find out what 100 is 1/5th of. So if you full price = x, then you would know that: 1/5x = 100 In an equation like that, how do you solve for x? The only thing you can do is divide by 1/5, so you end up with 100 divided by 1/5, which is 500. ---
**Author:** ![bengangmo](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@bengangmo](https://boards.straightdope.com/u/bengangmo)\ **Post date:** [October 23, 2013, 2:33am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/6 "2013-10-23T02:33:02Z")
Don’t care the 1/5 etc etc You pay 20% % means “per 100” Simply divide your price by 20, then multiply by 100. You pay 25%? Divide your price by 25, multiply by 100. NOTE: I’ve just gone through this with a less than math literate teenager - working percentages backwards, forwards and sideways. Talking about times 5, times 8, divide by 3 etc etc that seem intuitive for the math literate just induced a blank stare. The work our 1% method and then times 100 he got every time. ---
**Author:** ![urban1a](https://avatars.discourse-cdn.com/v4/letter/u/e47c2d/32.png) [@urban1a](https://boards.straightdope.com/u/urban1a)\ **Post date:** [October 23, 2013, 2:44am UTC](https://boards.straightdope.com/t/math-question-there-must-be-some-way/671997/7 "2013-10-23T02:44:49Z")
Amazing and everyone clearly understandable. Thanks, all. Bob