Graham's long-distance telephone plan includes the first 500 minutes per month in the $15.00 monthly charge. For each minute after 500 minutes, Graham os charged $0.02. Write a function that describes Graham's total long-distance charge in terms of the number of minutes he uses in a month.

I formed a function but it only works when the amount of minutes are 500 or greater:

f(x)=15+(m-500)(0.02); where m is the amount of minutes used in a month, 15 is the base fee, 500 is the allotted time in the plan, and 0.02 is the charge per minute after 500 minutes is up.

Question

Graham's long-distance telephone plan includes the first 500 minutes per month in the $15.00 monthly charge. For each minute after 500 minutes, Graham os charged $0.02. Write a function that describes Graham's total long-distance charge in terms of the number of minutes he uses in a month. 

I formed a function but it only works when the amount of minutes are 500 or greater: 

f(x)=15+(m-500)(0.02); where m is the amount of minutes used in a month, 15 is the base fee, 500 is the allotted time in the plan, and 0.02 is the charge per minute after 500 minutes is up.