Question

On your tenth birthday, you received 500 which you invested at 9.5% interest, compounded annually. your investment is worth $2500. how old are you today?

Answer 1

Compounded once a year: A=P(1+r)^t
2500=500(1+0.095)^t
1.095^t=5
because the unknown number is an exponent, use log to find the unknown:
log(1.095^t)=log5
tlog1.095=log5
t=log5/log1.095
use your calculator, t=17.734
so this person is about 28 years old.