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