Can you check these simple interest problems?

1. If you borrow $421 for nine years at an interest rate of 4%, how much interest will you pay?
Ans: I = prt
I = $421*.04*9
I = $151.56

2. If the balance at the end of 8 years on an investment of $630 that has been invested at a rate of 9% is $1083.60, how much was the interest?
Ans: I = prt
I = $630*.09*8
I = $453.60

or could it have been solved by subtracting?
$1083.60 - $630 = $453.60