72

If you want to work out how long it's going to take a country to double its income, you have to divide 72 by the country's average growth rate. Thus, at India's average annual growth rate of 1.8% in 1950-75, its economy would double in 72/1.8 = 40 years; China's, at 6% in 1975-2000, would double at a much foreshortened 72/6 = 12 years.

I also liked the one-size-fits-all coffee-cup lids as an example of the myriad kinds of rather bland, but cumulatively significant, kinds of innovation that characterise economic growth.