=number*(1-percent)

If you need to decrease a number by a certain percentage, you can use a simple formula that multiplies the number times the percent - 1.

In the example, the active cell contains this formula:

=C6 * (1 - D6)

In this case, Excel first calculates the result of 1 - the value in D6 (.2) to get 0.8 which it then multiplies times the value in C6 (70) to get a final result of 56:

=70 * (1 - .2)

=70 * (0.8)

=56

Note that must subtract the percent from 1 if you want to decrease a number. If you don't, you'll simply get the amount of the number that the percentage represents.