The cost to rent a bike at the beach is $6.75 an hour plus an insurance fee of $15. Marcia spent $48.75 when renting a bike on a recent trip to the beach. For how many hours did Marcia rent the bike?
Let X be the number of hours Marcia rented the bike.
The insurance fee is 6.75X + 15.
So Marcia paid 6.75X + 15 = $48.75.
Thus 6.75X = 48.75 - 15 = $33.75.
Thus X = $33.75 / 6.75 = <<33.75/6.75=5>>5 hours. Answer: \boxed{5}.