Deposit: K

1 year later: K*(1.04)

2 years later: K*(1.04)*(1.04) = K*(1.04)^2

3 years later: K*(1.04)^3

...

15 years later: K*(1.04)^15 =108,056.62

For me, the concept of "a formula" is very, very weak. Figure it out! Make your own formulae. The area of concentration you should pursue is in solving problems, rather than in memorizing things. Take a deep breath and think it through.