# Economics - Exponential and Logarithmic Functions

The rule-of-70 provides a simple way to calculate the approximate number of years it takes for the level of a variable growing at a constant rate to double. The rule states that if the constant growth rate is r, the variable will double after approximately n years when n = 70/ r.

a) Explain how this rule can be derived.

b) State and derive the corresponding rule-of-x, the approximate number of years it takes for a variable growing at a constant rate to treble.

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