Many modern calculators have a special button for converting hours, minutes and seconds into hours written as a decimal, and vice versa. The button is useful for calculating speeds in miles per hour when the time is given in terms of hours, minutes and seconds.

Suppose a car travels 100 miles in 2 hours 15 minutes and 30 seconds. We want to find the average speed in miles per hour. First convert 2 hours 15 minutes and 30 seconds into hours by pressing

2 |
15 |
30 |

Then press the equal button to obtain 2.2583 hours (to 4 decimal places). The average speed is then miles per hour.

Conversely suppose the car travels 100 miles at 35 miles per hour. It takes hours. We want to change this into hours minutes and seconds. To do this press

2.857 |

Then press the equals button to obtain 2 hours 51 minutes 26 seconds (to the nearest second).