Page 165 - ICSE Math 4

2. Subtract the following.
(a) 6 h 24 min 30 s from 8 h 16 min 15 s

(b) 3 h 15 min 5 s from 5 h 28 min 45 s

(c) 4 h 45 min 35 s from 5 h 19 min 15 s
(d) 12 h 35 min 52 s from 14 h 21 min 37 s

Elapsed Time

Elapsed me or dura on is the me that passes between the start and end of an event.
We can calculate elapsed me or dura on if we know the star ng and fi nishing me of an
event. It can be calculated using the following methods.

 By forward or backward coun ng
 By addi on or subtrac on

Let’s learn how to calculate elapsed me using these two methods.

By forward or backward coun ng

To calculate elapsed me or dura on by forward or backward coun ng, we can use a me
number line. Let’s consider an example to understand this.

Example 10: Manan started his homework at 6:45 p.m. and fi nished it at 8:45 p.m. How
much me did he take to fi nish his homework?
Solu on: Time when Manan started his homework = 6:45 p.m. (star ng me)

Time when Manan fi nished his homework = 8:45 p.m. (fi nishing me)

Step 1: Draw a number line and mark the star ng me and the fi nishing
me on it.

6:45 p.m. 8:45 p.m.
(star ng me) (fi nishing me)

Step 2: Count forward to the nearest hour from the star ng me. It will
be 7:00 p.m. Mark it on the number line and count the number of
minutes between 6:45 p.m. and 7:00 p.m.

15 min
6:45 p.m. 7:00 p.m. 8:45 p.m.

We can see that there are 15 minutes between the two mes.
Mark it on the number line.

Step 3: Count backward to the nearest hour from the fi nishing me. It will
be 8:00 p.m. Mark it on the number line and count the number of
minutes between 8:00 p.m. and 8:45 p.m.

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