Page 173 - ICSE Math 5

Addition, Subtraction, Multiplication and Division of Time

We can also perform the operations of addition, subtraction, multiplication and division on time.
Let's consider the following examples.

Example 7: Add 6 h 40 min 35 s and 3 h 25 min 45 s.
Solution: h min s
1 1
6 4 0 3 5
+ 3 2 5 4 5
1 0 0 6 2 0

In this example, 35 s + 45 s = 80 s which is more than 60 s.
So, subtract 60 s (i.e., 1 min) from 80 s and carry over 1 to the minutes column.

Similarly, 40 min + 25 min + 1 min = 66 min > 60 min.
So, subtract 60 min (i.e., 1 h) from 66 min and carry over 1 to the hours column.

So, 6 h 40 min 35 s + 3 h 25 min 45 s = 10 h 6 min 20 s.
Example 8: Subtract 8 h 38 min 29 s from 15 h 26 min 15 s.

Solution: h min s
715 (60 + 25 = 85)
8 5 6 15
1 4 2 5 7 5 (60 + 15 = 75)
1 5 2 6 1 5
– 8 3 8 2 9

6 4 7 4 6
In this example, 15 s – 29 s is not possible. So, borrow 1 min (i.e., 60 s) from the
minutes column and add to 15 s. Then subtract 29 s from it.

25 min – 38 min is not possible. So, borrow 1 h (i.e., 60 min) from the hours column
and add to 25 min. Then subtract 38 min from it.
So, 15 h 26 min 15 s – 8 h 38 min 29 s = 6 h 47 min 46 s.

Example 9: Multiply 3 h 14 min 9 s by 8.
Solution: h min s Remember
3 7 To multiply time,
3 1 4 0 9 first multiply seconds,

× 8 then minutes and finally
hours.
2 4 1 1 2 7 2

Product = 24 h + 112 min + 72 s = 24 h + 112 min + (60 + 12) s

= 24 h + 112 min + 60 s + 12 s
= 24 h + (112 min + 1 min) + 12 s (since 60 s = 1 min)
= 24 h + 113 min + 12 s = 24 h + (60 + 53) min + 12 s

= 24 h + 60 min + 53 min + 12 s

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