Hours and Minutes Study Guide

### It’s Not That Complicated!

Although hours and minutes computations may seem intimidating at first, once you get the hang of it, it really is not all that complicated. There are several methods that you can use to simplify the process of adding or subtracting time, but for our purposes, we are only going to discuss two of the methods that we feel are the easiest to grasp. One requires the use of a standard calculator, while the other does not.

### Hours and Minutes Conversion

The first method of computation requires that you convert hours to minutes or minutes to hours when necessary to add or subtract a common denominator. This method does not require the use of a calculator. Let’s say you are asked how many hours you were on duty if you checked in at 12:14 and were off duty at 18:08. In this case, you must subtract 12 + 14 (twelve hours and fourteen minutes) from 18 + 08 (eighteen hours and eight minutes). When subtracting, you must always remember that both the hours and minutes of the number you are subtracting from should be larger than the other number. In this example, if you try to do the subtraction, it would look like this:

• (18 + 08) – (12 + 14)

So, the hour number (18) is larger than the hour number (12), but the minute number (08) is smaller than the other minute number (14). So we must make it larger. How do you do that? You borrow some minutes from the hour! Since there are 60 minutes in an hour, we move 60 from the hour column and add it to the minutes. That reduces the hour column by exactly 60 minutes, or one hour. Now the problem looks like this:

• (17 + 68) – (12 + 14)

Now the subtraction is a simple matter. Just subtract 12 from 17 for the hours and 14 from 68 for the minutes. Your answer should be:

• 5 + 54

Here’s an example of adding two times together. In this example, another hours/minutes conversion will be required before performing the calculation. Let’s say you were asked to add together the following two duty periods:

• (5 + 56) +(6 + 17)

For addition problems, just add the two together, like any other math problem, but do not carry over anything. In other words, add hours to hours and minutes to minutes. Your answer should look like this:

• 11 + 73

Since eleven hours and seventy-three minutes makes no sense, we need to do some converting. Just as we did with the subtraction problem, we will move some minutes, but this time we will be moving from the minutes to the hours column. Again, since we know that there are 60 minutes in an hour, we will move 60 minutes from the minutes column back to the hours column increasing it by one hour and we will continue doing so until the answer makes sense (the minutes are less than 60). Here is what your answer should look like:

• 12 + 13

So that is basically how you would do every subtraction and addition problem you are faced with. Always remember that if things don’t look right, you need to move some minutes either from or to the hours column. You can practice a few of these on your own, or you can take the Hours and Minutes Test now and apply the rules we have taught you.

### The 940 Rule

• Our Method: (4 0 08) + (5 0 23)
• Equivalent to:( 4 + 08) + (5 + 23)

If you have been following along, your answer on the calculator to this point should be:

• 9 0 31 (which equates to 9 + 31)

That one was easy. Just as in the earlier problems you did without a calculator, you must look at the time and see if it makes sense. Here we have nine hours and thirty-five minutes. It does make sense, so that is your answer. But what if we had a problem that didn’t make sense? Let’s try the following one. Add 4 + 45 and 7 + 56. Again, here are the keys you would depress on the calculator:

• Our Method: (4 0 45) + (7 0 56)
• Equivalent to: (4 + 45) + (7 + 56)

If you have been following along, you answer on your calculator should be:

• 11101

It is very apparent that the number makes no sense. So here is where the “940 rule” comes into play. Add 940 to your answer until the number makes sense. In other words, when it looks like hours and minutes — with a (0) in between the hours and minutes. In this problem, you would add 940 only one time, because your first answer that comes up should look like this:

• 11101 + 940 = 12 0 41 (which equates to 12 + 41)

In some cases, when adding a series of times, you will be required to add 940 several times before the proper hours and minutes will display. Let’s look at another example. Let’s add 12 + 56 and 34 + 59 and 45 + 56. Here’s how it would look:

• Our Method: (12 0 56) + (34 0 59) + (45 0 56)
• Equivalent to: (12 + 56) + (34 + 59) + (45 + 56)

• 91171

Again, it is apparent that this is not in an hours and minutes format, so we have to add 940 until it makes sense. But in this case, it requires that we add it more than once. After the first entry, here’s what it will look like:

• 91171 + 940 = 92111

After the second 940 addition, we have something that looks right and this is your final answer:

• 92111 + 940 = 93 0 51 (which equates to 93 + 51)

The 940 rule also works well for subtracting. The only difference is that you must subtract 940 from your answer rather than add. Here’s an example. Subtract 14 + 34 from 23 + 12. Here’s how it would look:

• Our Method: (23 0 12) – (14 0 34)
• Equivalent to: (23 + 12) – (14 + 34)