If a nation's real GDP is growing at 2 per year its real output will double in approximately?

1) For a quick approximation, use the rule of 72: divide 72 by the interest rate to find the number of periods it will take. In this case, 72/2 is 36 years.

2) For an exact answer, you can solve this problem: (1+r)^t = N, where r is the growth rate, t is the number of time periods, and N is the ratio of the original you want. For this problem, you'd have (1.02)^t = 2, and solving for t, we get t = (ln 2)/(ln 1.02) [or you can use log in place of ln]. Solving, this is 35.003 years.