Cesar rents a bicycle for a flat rate plus $10 per hour. He models the cost of the rental with the function f(x)=10x+15 , where x is the number of hours he rents the bicycle, and f(x) is the total cost of the bicycle rental, including the flat rate and the hourly fee. What is the flat rate for the bicycle rental?

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The flat rate for the bicycle rental is $15. This can be found by looking at the function f(x) = 10x + 15 and identifying the constant term, which represents the flat rate.