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Determine the investment's original cost and the selling price of the investment. For example, a person purchases 100 shares of stock on January 1, 2005 for $40 each, then sells all 100 shares on December 31, 2009 for $65 a share. His original cost is $4,000 and his selling price is $6,500.

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Determine the rate of return on the investment by dividing the gain (the difference between the selling price and the original cost) by the original cost. In our example, $1,500 divided by $4,000 equals a return of 37.5 percent.

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Determine the number of years the investor kept the investment. In our example, the investor held the stock for five years.

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Divide the rate of return by the number of years the investor held the shares to calculate the average rate of return. In our example, 37.5 percent divided by 5 years equals 7.5 percent per year.