Arithmetic Grade Three

Lesson One hundred and thirteen: Rounding and Multiplying

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

Rules for rounding to the nearest 100:

  1. If the number in the tens column is 4 or less round down. Example: 342 -> 300.
  2. If the number in the ones column is 5 or more round up. Example: 451 -> 500.
Special Case:

If you have a number like 950 you have to remember that it will have to round up to 1000. Many times students are confused by this situation.

Rounding and then Multiplying:

Here's a sample problem for you: 

     
      Original          Rounded 
       Problem          Problem
        646      --->    600
        x 4      --->    x 4
        ----             ----
              ANSWER:   2400
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
        315      --->    300
        x 5      --->    x 5
        ----             ----
              ANSWER:   1500

Continue to next page of lesson