Let's start by recalling that the percent proportion is a proportion that sets the ratio of a part of a whole equal to the ratio of a percent out of 100.
part/whole=percent/100
We want to find the number of which 20 % is 75, which means that we want to find the whole. Let w represent the whole.
75/w=20/100
Now we can solve the proportion we got for w. Let's do this by using the fact that the cross products of any proportion are equal.
