Here’s how I do it. I call it “carrying the sixes”. The great advantage is that it works EXACTLY like regular arithmetic, except you have to remember that instead of “carrying the tens”, you have to “carry the sixes”.
Let’s try adding first. Something really simple, like 1 hour and 56 minutes plus 2 hours and 57 minutes.
1:56
+ 2:57
------
First you add the 6 and 7, making 13. But you can’t write “13”, because we do only one digit at a time. The highest digit allowed in the “ones” column is “9”, so write down the “3” and “carry” the remaining 10.
1
1:56
+ 2:57
------
3
Now we’re working on the “tens” column, so count the 10 as a “1”. Add the 1 and 5 and 5, giving you eleven. But you can write “11”, because we do only one digit at a time. If this was regular numbers, the highest digit would be “9”, but we are limited to the number of minutes in an hour, so the highest digit is “5”. So write down the “5” and “carry” the remaining 6.
BUT, Since we are crossing to the other side of the “:” sign, to the “hours” column, you have to remember that there are sixty minutes in an hour. So the carrying is of 60 minutes in our case, which is a “6” in the “tens of minutes” column, but a “1” in the “units of hours” column.
1 1
1:56
+ 2:57
------
53
Then just add up the hours, and you find that “1:56” plus “2:57” makes “4:53”.
The same applies if your times had included seconds, like “4:34:23”. You just have to remember that when you cross over the “:”, you have to use a “6 to 1” conversion, instead of the “10 to 1” that we usually use. The same logic is used if you were adding days, like “3 days, 15 hours, and 36 minutes” – When you cross over between hours and days, you need a “24” conversion factor.
I think that for many types of problems, my method is simpler than the others mentioned above. This is especially true if you have a list of three or more times that you need to add up.
You have to choose the best tool for the problem at hand. I chose the example above because I wanted the minutes to be over a hundred, to illustrate the “carrying”. But really, the easiest way of adding those particular times would be to say that “Four minutes less than two hours, plus three minutes less than three hours, makes seven minutes less than five hours, which is 4:53”.
Subtraction works the same way. Just remember to carry “6” instead of carrying “10”
4:53
- 2:57
------
You can’t take 7 from three, so you need to take 10 minutes from the “tens of minutes” column, and add it to the “units of minutes” column. The “5 and 3” becomes “4 and 13”. Then you can take 7 from 13:
4 : 4 13
- 2 : 5 7
---------
6
You can’t take 5 from 4, so you need to take an hour from the “hours” column, and add it to the “tens of minutes” column. But you have to remember than one hour is worth SIX tens-of-minutes. Adding that to the 4 already there gives 10.
3 :10 13
- 2 : 5 7
---------
6
The rest is easy. Ten minus five is five. Three minus two is one.
3 :10 13
- 2 : 5 7
---------
1 : 5 6
Result: 4:53 minus 2:57 equals 1:56.
What could be easier?
Homework assignment: Multiply 1 hour and 16 minutes by 8.