Addition and subtraction of time periods require us to follow some simple guidelines. Let’s look at addition of time periods first.
Addition of time periods
When we add two time periods, we add the seconds first, then the minutes, and finally the hours. If the time period of seconds and minutes are more than 60 after addition, we take away 60 from the respective numbers, and add one more unit to the higher time unit.
Let’s look at 3 examples:
Example 1
| 2 hours | 32 minutes | + |
| 4 hours | 15 minutes | |
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| 6 hours | 47 minutes | |
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Example 2
| 7 hours | 24 minutes | + |
| 8 hours | 42 minutes | |
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| 15 hours | 66 minutes | |
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But 66 minutes = 60 + 6 minutes = 1 hour + 6 minutes.
So we increase the hours by 1, and keep the minutes as 6. The answer then is 16 hours 06 minutes.
Example 3
| 4 hours | 27 minutes | 36 seconds | + |
| 1 hour | 32 minutes | 42 seconds | |
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| 5 hours | 59 minutes | 78 seconds | = |
| 6 hours | 0 minutes | 18 seconds | |
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Here 78 seconds is more than 60, so we increase the minutes by 1, and have 18 seconds remaining. When we add 1 minute to 59 minutes, that becomes 60 minutes, or exactly 1 hour, so we increase the hours by 1, and have 0 minutes.
Hence the answer of 6 hours, 0 minutes and 18 seconds.
Subtraction of time periods
When we subtract two time periods, we first check whether the time period in the first one is more or less than the second time period. It the first one is smaller than the second, we trade in one unit of time from the higher time unit.
For example, when we have to subtract 12 seconds from 1 minute and 10 seconds, we notice that 10 seconds is lesser than 12 seconds, so we borrow one unit (in this case 1 minute = 60 seconds) and add to 10 seconds. So we get 70 seconds, and now we can subtract the two numbers.
70 – 12 = 58 seconds is the final answer.
Now let’s look at some more examples of subtraction of time periods:
Example 1
| 6 hours | 24 minutes | – |
| 2 hours | 12 minutes | |
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| 4 hours | 12 minutes | |
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Example 2
| 6 hours | 24 minutes | – |
| 1 hour | 36 minutes | |
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Note that we cannot subtract 36 from 24, hence we trade 1 hour (= 60 minutes) and add to 24 minutes. The question now becomes –
| 5 hours | 84 minutes | – |
| 1 hour | 36 minutes | |
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| 4 hours | 48 minutes | |
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Example 3
| 5 hours | 4 minutes | 0 seconds | – |
| 3 hours | 42 minutes | 10 seconds | |
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Notice we can’t subtract 10 seconds from 0 seconds, and 42 minutes from 4 minutes. So we need to borrow on unit each from the higher units to the left. So we rewrite the first time period as –
| 5 hours | 4 minutes | 0 seconds | or |
| 4 hours | 64 minutes | 0 seconds | or |
| 4 hours | 63 minutes | 60 seconds |
Now we can solve the question as below:
| 4 hours | 63 minutes | 60 seconds | – |
| 3 hours | 42 minutes | 0 seconds | |
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| 1 hour | 21 minutes | 50 seconds | |
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Here are some more examples of addition and subtraction of time periods.