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Addition and Subtraction of Time I

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

6 hours 47 minutes

 

Example 2

7 hours 24 minutes +
8 hours 42 minutes

15 hours 66 minutes

 

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

5 hours 59 minutes 78 seconds =
6 hours 0 minutes 18 seconds

 

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

4 hours 12 minutes

 

Example 2

6 hours 24 minutes
1 hour 36 minutes


 

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

4 hours 48 minutes

 

Example 3

5 hours 4 minutes 0 seconds
3 hours 42 minutes 10 seconds


 

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

1 hour 21 minutes 50 seconds

 

Here are some more examples of addition and subtraction of time periods.