The price of a math book after a discount of 25% is $36. What is the original price of the math book?

If $36=75%,

That means we can divide both sides by three to get $12=25%,

And because 25x4 is 100, we multiply both sides of the equation by 4 to get $48=100%,

So the answer is $48 dollars.

This also worked for me but I'm not sure it's the most reliable way:

36 : 0.75 = 48

(amount after discount : percentage that is remaining)