Arithmetic Grade Three

Lesson Thirty-two: Rounding and then Subtracting

Before we get too far into this lesson, let's review a little:

Rules for rounding to the nearest 10:

  1. If the number in the ones column is 4 or less round down. Example: 42 -> 40.
  2. If the number in the ones column is 5 or more round up. Example: 45 -> 50.
Special Case:

If you have a number like 495 you have to remember that it will have to round up to 500. Many times students are confused by this situation.

Rounding and then Adding:

Here's a sample problem for you: 

     
      Original          Rounded 
       Problem          Problem
         86      --->     90
        -72      --->    -70
        ----             ----
              ANSWER:     20
In this example, you will notice that we first round off the two numbers and then we subtract them. This is a real good way of estimating an answer. It is much more difficult to subtract two un-rounded numbers in your head than to subtract these two rounded numbers. By rounding we help ourselves to be able to get a rough answer without needing to write anything down. The actual answer to 86-72 is 14. Our rounded answer is 20. That's a pretty close estimate.

Here's another example for you: 

     
      Original          Rounded 
       Problem          Problem
         85      --->     90
        +51      --->    +50
        ----             ----
              ANSWER:     40

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