Question

Annie's reading class begins at 8:35 and lasts 1 hour and 45 minutes. What time is her reading class over? Show two different ways to find the answer.

Answer 1

10:20

Explanation:

The ending time of the class is found by adding the duration to the starting time.

We use a base-60 number system for recording time and angles in hours/degrees-minutes-seconds. There are 60 seconds in a minute, 60 minutes in an hour or degree.

When the sum of minutes is more than 60, we convert 60 of those minutes to an hour.

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Solution

The ending time of the class is the sum ...

(8 hours, 35 minutes) + (1 hour, 45 minutes)

= (8 +1 hours, 35 +45 minutes)

= (9 hours, 80 minutes)

= (9 hours, (1 hours + 20 minutes))

= (10 hours, 20 minutes)

The class is over at 10:20.

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Alternate solution

As in base-10 arithmetic, sometimes addition is easier using the "complement" of a number.

(8 hours, 35 minutes) + (1 hour, 45 minutes)

= (8 hours, 35 minutes) + (2 hours, -15 minutes)

= (8 +2 hours, 35 -15 minutes)

= (10 hours, 20 minutes)

The class is over at 10:20.