Telling Time — Subtracting Time
How do you subtract time?
When subtracting time you do not look at each digit. You look at the minutes as a whole. In the problem below you cannot subtract 52 minutes from 15 minutes (15 − 52 = ???), because 15 is smaller. You must borrow.
| 5 hours 15 minutes |
| − 1 hour 52 minutes |
| ??????????????? |
How do you do this? You go to your hours and take 1 away. (5 − 1 = 4 hours). You now have 4 hours.
An hour equals 60 minutes. You add the 60 minutes to the 15 minutes. (60 + 15 = 75 minutes). This gives you 75 minutes. Your problem now looks like this:
| 4 hours 75 minutes |
| − 1 hour 52 minutes |
Now you are able to subtract:
| 4 hours 75 minutes |
| − 1 hour 52 minutes |
| 3 hours 23 minutes |
The answer is 3 hours and 23 minutes.
- Borrowing and carrying in regular math means exchanging 10 ones for a ten (or 10 tens for a hundred, etc.).
- Borrowing and carrying in time math means exchanging 60 minutes for an hour.
You follow the same rules when you are trying to figure out what time it was so many hours and minutes ago.
Example 1
What time was it 3 hours and 45 minutes ago?
Turn this into a subtraction problem. The clock says it is 8:25. Subtract 3:45 from 8:25.
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Now you are able to subtract. The answer is 4:40. Three hours and 45 minutes before 8:25 was 4:40.
Example 2
What time was it 3 hours and 50 minutes ago?
The clock says it is 1:45. Subtract 3:50 from 1:45.
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It is easy to subtract the minutes: 105 − 50 = 55. But 2 − 3 cannot be subtracted directly. Look at the clock — the hour hand is on the 1. Back the clock up 3 hours from 1 and you get 10. Add this to your minutes.
The answer is 10:55.