For a single payment with compound interest, the equation to use is F=P(1+i)^n where F is the value after n periods, P is the present value, and i is the interest rate.
If we want the final value F to double in 5 years, F is then equal to P then n=5. The equation is now:
2P=P(1+i)^5
2=(1+i)^5
i=14.87% per year
