Arithmetic Grade Three

Lesson One hundred and six: Rounding and Multiplying

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 Multiplying:

Here's a sample problem for you: 

     
      Original          Rounded 
       Problem          Problem
         66      --->     70
        x 4      --->    x 4
        ----             ----
              ANSWER:    280
By rounding off the top number we can quickly multiply and get an answer which is close to the actual answer. This is a quick way of estimating the real answer.

Here's another example for you: 

     
      Original          Rounded 
       Problem          Problem
         35      --->     40
        x 5      --->    x 5
        ----             ----
              ANSWER:    200

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