Given:
amount borrowed: 1,058
interest rate: 13%
term: 9 months
I'm assuming that the 13% rate is the annual interest rate. I'm also assuming that this problem is a simple interest problem.
Interest = Principal * rate * term
Interest = 1,058 * 13% * 9/12
Interest = 103.16
Total payment = Principal + interest
Total payment = 1,058 + 103.16 = 1,161.16
His monthly payment will be:
1,161.16 / 9 mos = 129.02
Devin will pay a 129.02 per month for 9 months to pay off his loan principal of 1,058 and the interest of 103.16.