I can't understand this equation, Can you explain it for me?

A variable growing at some exponential rate doubles over fixed intervals of time.

The doubling time is given by the rule of 722

, which states that a variable's

approximate doubling time equals 72 divided by the growth rate, stated as a whole

number. If the level of income were increasing at a 9% rate, for example, its

doubling time would be roughly 72/9, or 8 years. Notice the use of the words roughly

and approximately. The actual value of an income of $1,000 growing at rate r for a

period of n years is $1,000 (1 + r)

n

. After 8 years of growth at a 9% rate, income

would thus be $1,000 (1 + 0.09)8

= $1,992.56. The rule of 72 predicts that its value will be $2,000. The rule of 72 gives an approximation, not an exact measure, of the

impact of exponential growth

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