$48
Step-by-step explanation:
Discount = 25% = 0.25
The final price = $36
Percentage of the original price = 100% -25% = 75%
Percentage of the original price = 0.75
let the original price = y
Percentage of the original price × original price = 36
0.75 × y = 36
0.75y = 36
The original price of the math book = $48