20+ .05(t-300) = c
The equation includes the basic charge or $20 that is not dependent upon anything. Then the equation has you determine how many minutes will be charged by subtracting the 300 provided minutes from t. t represents the total number of minutes. Once the subtraction is completed, you will multiply the number of minutes by the 5 cents (,05), adding in the 20 for total cost for the month.