*No calculators allowed, You won’t need them. All you will need is simple arithmetic (the ability to multiply things by two, and to divide) and the rule of 72—the fact that a quantity growing at 1% per year doubles in 72 years, a quantity growing at 2% per year doubles in 36 years, a quantity growing at 0.5% per year doubles in 144 years, et cetera, a quantity growing at 1% per year grows by 41.41% in 36 years, etc…*

PART 3 (25 MINUTES): Calculations

Consider the 144 years between 1866 and 2010…

- World population grew from about 1.1 billion to 6.4 billion. About how many doublings did population undergo? What is the average growth rate of world population over those 144 years?
- World total real GDP (on one somewhat arbitrary set of assumptions, at least) grew from $875 billion of today’s dollars in 1866 to $56 trillion of today’s dollars today. What was the average growth rate of world total real GDP over those 144 years?
- If world total real GDP growth continues to grow at its average 1866-2010 rate, what will world total real GDP (in today’s dollars) be in 2155? Do you think this is possible? Reasonable? Absurd? Why?
- By contrast, total world GDP (on one somewhat arbitrary set of assumptions, at least) grew from $437.5 billion of today’s dollars in 1794 to $875 billion of today’s dollars in 1866. Suppose total world real GDP growth had continued at its 1794-1866 pace from 1866-2010. What would total world real GDP be today?

If I follow correctly. 2. implies 6 doublings in 144 years or an annual growth rate of 3%.

4. implies a growth rate of 1% (1 doubling in 72 years) continuing at that pace for an other 144 years the economy would have experienced two doublings to 3.5 trillion, not even 10% of today's size. Amazing!

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