You can rent books at a local store for $5.00 each. An Internet company offers a flat fee of $20.00 per month for as many books as you want. How many books do you need to rent to make the online rental a better buy?

The correct answer is: We need to rent more than 4 books to make the online rental a better buy.

Step-by-step solution:-
Let the number of books to be bought in a month be x.
∴ Total cost incurred for renting at local store = Number of books * cost per book
∴ Total cost incurred for renting at local store = x × $5 ..................................................... (From given information)
∴ Total cost incurred for renting at local store = 5x ........................................................... (Equation i)
Also,
Total cost incurred on internet company for a month = $20 .................. (From given information) ..................... (Equation ii)
We need to find the number of books to be rented to make the online rental a better buy.
i.e. we need to find the value of x for which-
Cost incurred at local store > Cost incurred on online rental
∴ 5x > 20 ...................................................................................... (From Equations i & ii)
∴ x > 20/5 .................................................................................. (Dividing both sides by 5)
∴ x > 4