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.