Rylan paid a total of $4,120.00, including principal and interest, over an 18-month period, to pay off his credit card. Choose the equation that could be used to solve how much, on average, he paid in addition to the $172.00 average minimum monthly payment.
a) (172 + x)18 = 4,120
b) 172x + 18 = 4,120
c) 18x + 172 = 4,120
d) 172(18 + x) = 4,120