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Calculations with the Clock

When you do calculations with time, it’s important to remember that there are 60 seconds in one minute and 60 minutes in one hour. The place value system changes a bit, but not a lot. You just have to make sure that when you change hours into minutes, one hour is 60 minutes. If you want to change from minutes to hours, remember that 60 minutes goes into one hour. Let’s check out some examples.


Subtracting Times from Each Other

You borrow


when you borrow from the number on the left side of the :-sign.

When you calculate time using the column method, you’ll do it in the same way as before. The only difference is that if you need to borrow, you borrow 6 instead of 10 when you borrow from the number to the left of the :-sign.

Example 1

How much time is between 08:30 and 09:45?

If you treat the times as numbers, you can do calculations with them in almost the same way you’ve learned before:

09:45 - 08:30 = 01:15 solved using the column method

You can see that there’s 1 hour and 15 minutes between 08:30 and 09:45.

Example 2

How much time is between 12:15 and 14:05?

Remember that there are 60 minutes in an hour.

14:05 - 12:15 = 01:50 solved using the column method

You can see that there is 1 hour and 50 minutes between 12:15 and 14:05.


Adding Times Together

When you want to add two times together and end up with more than 6 in a column, you put


above the column to the left.

When you use the column method to calculate time and need to add times together, you use the exact same method as before. The exception is when you get more than 5 when adding together the column to the right of the :-sign. Then you have to put a 1 over the column to the left, and the difference between the number and 6 underneath the line. This 1 is added to the column it sits above as usual.

Example 3

What’s the time 2 hours and 15 minutes after 16:50?

Remember that when the sums of the columns become greater than 5, you need to start counting from 0 again and put a 1 above the column to the left. This is because you count to 60 minutes before you start a new hour.

16:50 + 02:15 = 19:05 solved using the column method

2 hours and 15 minutes later, the time is 19.05.

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