Ellie and Gabby decided to buy 100 lawn gnomes, so they could spread them all over the neighborhood when everyone was asleep. They had to borrow $348 at a monthly interest rate of 4.2% and it took them 3 years to pay it off. How much did they pay in total? a)$996.42
b)$1200.69
c)$394.64
d)$393.72

(if you can tell me the name of the formula you use in this kind of problem id appreciate it so much!!!)