Answer:
b = 13
Step-by-step explanation:
the value of the discount is a constant, while the total cost of the books is a variable because it depends of the purchase
We can write this equation
5b - 7 = 58
Solve it
1) add 7 at both members
5b -7 + 7 = 58 + 7
5b = 65
2) divide both membera by 5
b = 13
