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Ernie is training for a marathon. Each month his running time on a 10-mile course improved by 10 percent over the previous month. It took Ernie 1 hour 40 minutes to complete the course the first month. How long did it take him the third month?

Explanation

Saying that his time improved by 10% each month means that he took 10% less time than the previous month. Start by writing his time for the first month in minutes.

1 hour 40 minutes = 60 minutes + 40 minutes = 100 minutes

To find 10% of a number, just move the decimal point 1 place to the left. 10% of 100 minutes = 10 minutes. So, in the second month his time was 100 – 10, or 90 minutes. Next find 10% of 90 minutes (move the decimal point 1 place to the left): 10% of 90 minutes = 9 minutes. So, in the third month his time was 90 – 9, or 81 minutes. Since 81 minutes = 60 minutes + 21 minutes, his time for the third month was 1 hour and 21 minutes.

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