Arithmetic Grade Three

Lesson Twenty-nine: Rounding and then Adding

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
         66      --->     70
        +72      --->    +70
        ----             ----
              ANSWER:    140
In this example, you will notice that we first round off the two addends and then we add them up. This is a real good way of estimating an answer. It is much more difficult to add two un-rounded numbers in your head than to add 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 66+72 is 138. Our rounded answer is 140. That's a pretty close estimate.

Here's another example for you: 

     
      Original          Rounded 
       Problem          Problem
         35      --->     40
        +51      --->    +50
        ----             ----
              ANSWER:     90

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