If you borrow $12,000 for 30 months at 6.5% simple interest, what is the total amount you will have to repay?

From your statement, i can assume that the interest 6.5% is for yearly.

30 months = 2.5 years

1 year interest for 100           = 6.5
1 year interest for 12000       = 6.5/100 * 12000
                                              = 780

2.5 years interest for 12000  = 780*2.5
                                              = 1950

so the total amount that you have to pay 
12000 + 1950
$ 13950