The function given is \( f(x) = 10x + 15 \), where \( x \) is the number of hours rented and \( f(x) \) is the total cost.
In this function:
- The term \( 10x \) represents the cost per hour ($10 per hour).
- The constant term \( 15 \) represents the flat rate.
Therefore, the flat rate for the bicycle rental is $15.