Why are you dividing the 2.5% by 15? It’s already an annual rate and you’re compounding annually. That’s one mistake. The other mistake is your exponent. The exponent is the number of compound periods, which is 15. Finally, what you are computing with your formula is not the interest; it’s the future value amount. You have to subtract off the original principal.

the number of times you compound would not make much difference at such a low interest rate. Even with continuous compounding your accumulated amount would be $4427.54 making $1384.54 in interest.

Think about it this way. At 0% interest, the number of times you compound a year would have no difference on the final amount.