Let me help you, but first I will explain the formula used to calculate monthly compound interest.
The formula used to calculate compound monthly compound interest is
a=p(1+r/n)^nt
P Represents the principal
R Represents the rate (in decimal)
N is basically 12 months
T Represents the time (in years)
Let's plug the numbers in.
First statement
"His bank has offered him a loan at 13% interest for 36 months"
12000(1+.13/12)^(12)(3)
12000(1+.13/12)^(12)(3) = $17,686.64
Second statement
"12% interest for 60 months"
Plug in the numbers into the formula.
12000(1+.12/12)^(12)(5)
12000(1+.12/12)^(12)(5) = 21,000.36
Answer
The answer would be "13% interest for 36 months" as it is much lower compared to the other statement.