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.
