Can you check these simple interest problems?

Question

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

Answer 1

First one is correct

For the second, yes, you could have just subtracted the two.
They actually gave you more information than was needed. Obtaining the same answer confirms that your answer is correct.

The second question would have been nice had it been worded this way:
If the balance at the end of 8 years on an investment of $630 that has been invested at a rate of i % is $1083.60, what was the interest rate ?

or

If the balance at the end of n years on an investment of $630 that has been invested at a rate of 9% is $1083.60, how many years did it take ?