Jack borrowed a sum of 12000$ from a financing company for 3 years at 15% per annum. What is the total interest he paid in 3 years?

So if this is simple interest which I'm assuming it is then this is how to find it:

-divide 12000 by 100 to get 120, which is 1% of 12000.

-multiply 120 by 15 to get 1800, which is 15% of 12000.

-multiply 1800 by 3 to get 5400, which is interest for 3 years!

Simple interest:

i = p r t

Here i = $12000 (1.15) (3) = $18250.50