The "rule of 70" is used to estimate:

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The number of years it takes for a variable to double at a given growth rate. The rule of 70 is a quick estimation tool: divide 70 by the annual growth rate (as a percentage) to approximate how many years it takes for a variable to double. For example, if an economy grows at 2% per year, it will take approximately 70 / 2 = 35 years for real GDP to double. At 7% growth, GDP doubles in about 10 years. This simple rule highlights the power of compound growth.

The rule of 70 has nothing to do with the natural rate of unemployment, optimal tax rates, or the spending multiplier. It is purely a mathematical shortcut for exponential growth calculations. The actual formula for doubling time is ln(2) / ln(1 + r), where r is the growth rate as a decimal, but 70 divided by the percentage rate provides a convenient approximation.

This concept is important for the CLEP exam because it illustrates why seemingly small differences in growth rates have enormous long-run consequences. An economy growing at 3% doubles in about 23 years, while one growing at 1% takes 70 years  -  a difference that compounds dramatically over generations. This is why policies that increase the long-run growth rate by even a fraction of a percentage point can have profound effects on living standards over time. The rule of 70 helps make these abstract growth comparisons concrete and intuitive.

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