# What is the equation for finding a number that is X% less than another number?

I know this seems like a juvenile question, but for some reason I can't recall how to do this.

Lets say I want to find 87 reduced by 99.75%. What is the equation to do that?

Thanks!

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See my answer to this similar question here math.stackexchange.com/questions/14715/… – Bill Dubuque Dec 20 '10 at 17:24

A simple formula for this is: $$\text{ New Number } = \text{ Old Number } \times \left(\frac{100 - x}{100}\right)$$

In general if a number is increased or decreased by $x\%$ then the new number is given by: $$\text{ Old Number } \times \left(\frac{100 \pm x}{100}\right)$$

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This is what I remember as the formula I was looking for! Thanks – Dream Lane Dec 20 '10 at 19:45

If you want $X$% of 87, multiply the latter by $\frac{X}{100}$. (So, for example, to get 25 percent, you multiply by $0.25$).

If by "reduced by 99.75%" you mean 99.75% of the total, then, multiply by $.9975$. If "reduced by 99.75%" means taking 99.75% off (so you are left with only 0.25%), then multiply by $0.0025$.

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Great answer! Very informative, thank you – Dream Lane Dec 20 '10 at 19:45