If 80% of a number is 100 what is 20% of that same number

We can rewrite these percentages in decimal form. That what we can write an equation to solve. A percentage is a decimal that is multiplied by 100. So to reverse that, let us divide by 100:

and 

So we know that 80% = 0.8, and 20% = 0.2.

An equation we can write for the question is the following.

, where x is the 'mystery' number.

So let us then solve for x:



So now we know that x = 125. Let us then create a second equation and solve the question:

, where y is the answer that the question is looking for.

Let us then solve for y:

.

So now we know that the answer to the question is 25.