What number is 15% of 80?
use percent proportions

!5% is equivalent to 15 parts over the whole 100 part. It is written in proportions as 15/100. 

Similarly, you need to find how many parts in 80 are equivalent to 15%or 15/100. So, just equate both equation with x as an unknown.

x/80 = 15/100
x = (15/100)*80
x = 12

Thus, the 15% of 80 is 12.