Question

If you invest $100 into an account that earns 9% interest each year, how many years will it take for you to have $200?

Answer 1

It does not say simple or compound interest.
Simple interest is rarely used these days, so assume compound.
Use the standard formula:
future value = present value*(1+rate/n)^(nt)
n=number of times interest is compounded per year (=1)
t=number of years
Plugging values,
200=100(1.09)^t
1.09^t = 2
take log
t(log(1.09))=log 2
t=log(2)/log(1.09)=0.6931/0.08618=8.04 years.