The solution would be like this for this specific problem:
Given:
25% discount
Hasan: d - 0.25d
Bella: 0.75d
d = original price.
0.25d = discount.
0.75d = discounted price.
Let’s say the price of the book is $1,000.
0.25d = 0.25(1000) = 250
0.75d = 0.75(1000) = 750
d - 0.25d = 1000 - 250 = 750
0.75d = 0.75(1000) = 750