If a country starts off as rich as the United States, with a GDP per capita of $46,000, and if GDP per capita grows 3% per year, then how many years will it take before GDP per capita is $1,000,000 per year?

If a country with a GDP per capita of $4,000 at its start grows at 8% per year how many years will it take before GDP per capita is $46,000?

If you wanted to double $1,000 in 10 years’ time, what average rate of return would you require on your investment?

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* What about technological knowledge? North Korea has access to most of the world’s technological knowledge and is able, for example, to build sophisticated weapons—perhaps even a nuclear bomb—thus differences in technological knowledge explain probably only a small fraction of the differences in the wealth of nations.

Increases in technological knowledge, however, are clearly important for growth at the world level (as opposed to explaining differences in wealth across nations)—as we will discuss at greater length in the next chapter.