Question: Looking back at Table, how many years will it take the low- and middle-income countries to double their per capita income, assuming they continue to grow at the average annual rate which prevailed during 1970-2005? How many years will it take for the high-income economies to double their per capita income, using their average annual growth rate for 1970-2005? How long would it take Sub-Saharan Africa to double its income per person if it grew at the average annual rate which prevailed from 1970-2005?

(Hint: using the Rule of 70, the doubling time in years = 70/annual % change; in finance, the Rule of 72 is often used in the same way. In fact, the Rule of 70 gives a very good estimate of doubling for percentage rates of 5 percent or less. The Rule of 72 gives better estimates for percentage changes between 5 and 10 percent. If growth rates are negative, what is being determined is the time to halve a value, and for negative values the Rule of 70 is more accurate.)