December 6, 2009 7:25 PM Subscribe

How do you find a number that minus a percentage of itself is a certain number?

What would be the formula (in a spreadsheet like pages.app) to find that 100 - 20% = 80, if you only know the percentage and the result (in this example 20% and 80)

I've tried googling this but I don't think I know the name or term of what I'm looking for, reverse percentage doesn't seem right...
posted by JulianDay to Education (7 answers total) 3 users marked this as a favorite

What would be the formula (in a spreadsheet like pages.app) to find that 100 - 20% = 80, if you only know the percentage and the result (in this example 20% and 80)

I've tried googling this but I don't think I know the name or term of what I'm looking for, reverse percentage doesn't seem right...

To be more precise...

X * (1 - 0.2) = 80

X = 80 / (1 - 0.2)

posted by mpls2 at 7:29 PM on December 6, 2009

X * (1 - 0.2) = 80

X = 80 / (1 - 0.2)

posted by mpls2 at 7:29 PM on December 6, 2009

An easier way is to remember that subtracting a percentage is the same as multiplying by a fraction. In the case of 20%, that is 8/10 (x-20% = x * 8/10).

That makes the problem much easier to solve.

posted by Deathalicious at 7:33 PM on December 6, 2009

That makes the problem much easier to solve.

- x - 20% = 80
- 0.8x = 80
- x = 80 / 0.8

posted by Deathalicious at 7:33 PM on December 6, 2009

Use this to turn the word problem into an equation:

"of" is * (multiplication)

"is" is = (the equality sign)

Example:

$43 is 90% (10% off) of what?

43 = 0.90 * x

x = ...

posted by msittig at 7:41 PM on December 6, 2009

"of" is * (multiplication)

"is" is = (the equality sign)

Example:

$43 is 90% (10% off) of what?

43 = 0.90 * x

x = ...

posted by msittig at 7:41 PM on December 6, 2009

I was going to work it all out, but then just thought, hey, why not work it out as a normal algebraic statement and feed it to wolfram alpha.

http://www.wolframalpha.com/input/?i=x+-+%28x*%2820%2F100%29%29+%3D+80

posted by TravellingDen at 7:46 PM on December 6, 2009 [1 favorite]

http://www.wolframalpha.com/input/?i=x+-+%28x*%2820%2F100%29%29+%3D+80

posted by TravellingDen at 7:46 PM on December 6, 2009 [1 favorite]

I may have learned only one thing in middle school math, but it was this formula:

"Percent over 100 = 'is' over 'of'."

Cross multiply and divide to solve for the unknown.

posted by eafarris at 6:05 AM on December 7, 2009

"Percent over 100 = 'is' over 'of'."

Cross multiply and divide to solve for the unknown.

posted by eafarris at 6:05 AM on December 7, 2009

This thread is closed to new comments.

X = 80 / 0.8

posted by mpls2 at 7:28 PM on December 6, 2009