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.