My Math Forum Calculate x as a % of y?

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 July 29th, 2011, 09:52 AM #1 Newbie   Joined: Jul 2011 Posts: 5 Thanks: 0 Calculate x as a % of y? Hi, can anyone tell me how to calculate x as a % of y? For example What % is 31 million of 72 million, for example? Thanks guys, sorry if this sounds dumb! I'm learning! Z
 July 29th, 2011, 01:06 PM #2 Senior Member   Joined: Jul 2011 Posts: 227 Thanks: 0 Re: Calculate x as a % of y? What's 1% of 72 million?...
 July 29th, 2011, 08:13 PM #3 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 463 Math Focus: Calculus/ODEs Re: Calculate x as a % of y? I would rewrite the question as follows: What percentage of y is x? Then translate the question into an equation as follows: "What" is z (the unknown) "percentage" is /100 "of" is · (multiplication) "y" is y "is" is = ("is" is short for "is equal to") "x" is x Now, putting it together, we have: (z/100)·y = x Solve for z by multiplying both sides by 100/y z = 100·(x/y) So we see that when we want to know what percentage x is of y, we simply divide x by y then multiply by 100.
 July 30th, 2011, 08:12 AM #4 Senior Member   Joined: Jul 2011 Posts: 245 Thanks: 0 Re: Calculate x as a % of y? You could also think: $\frac{100x}{y}%$ Percentage is actually an 'operator'. . . I didn't realize this (I was very lackadaisical in my intuitive side of math. . .) until a while back. Basically, a percentage sign means: $%=\frac{1}{100}$ This is because of the equality: $k%=\frac{k}{100}$ Interestingly enough, it turns out that percentage is--like all mathematical operators--a convention of sorts to simplify an expression. I think that's pretty cool--the entire world uses a mathematical operator without necessarily knowing what it means. xD
 July 30th, 2011, 10:19 AM #5 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,596 Thanks: 938 Math Focus: Elementary mathematics and beyond Re: Calculate x as a % of y? 31/72 ? .4305 ? 43.05%.
 July 30th, 2011, 08:11 PM #6 Senior Member   Joined: Apr 2011 From: USA Posts: 782 Thanks: 1 Re: Calculate x as a % of y? I look at this in a much different manner. I find most people don't have too much trouble learning how to find x% of some other number, which is just a multiplication. i.e. what is 10% of 2100: 2100 * .10 = 210. (And refer to CherryPi's post about why you're moving that decimal over twice. A lot of people seem not to realize that a percent means parts out of 100.) That is giving us what is 10% of the 2100. If you already have the 210 and want to know what percent that is of the 2100, it means you have the "answer" to the above equation and it becomes 2100 * x = 210. The percent is missing. If we multiply to go forward, we start with the "answer" and divide to go backwards: 210/2100 = .10. (That will give the decimal version. You have to move it back over twice to put it back into a percent form of 10%.) So any time you want to know what percent x is of y, divide in that order: x/y. If you have the 210 and the 10% and you want to know what the original number was, then that's like this: x * .10 = 210. Again, to go backwards we divide: 210/.10 = 2100. So if you already have the portion of some other number and you want the original number, that is, x is y percent of what original number, again you divide in that order. Or you can learn it this way: Base = 2100, Rate = 10% and Part = 210. When doing a division, the part always come first in the division.

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