Some experts claim that for the next 30 years the population growth in China will average 0.35% per annum and that its economy will average a growth rate of 6.85%. This implies that the income per capita of the average Chinese citizen should grow every year at the rate of (6.85%-0.35%) = 6.5%.

Given the above information estimate how often (every how many years) would it double?

a): every 7 years b): every 9 years c): every 11 years d): every 29 years e): other