Arithmetic Grade Four

Lesson Thirty: Rounding and then Adding

Review of rules for rounding to nearest 100:
  1. If the number in the tens column is 4 or less, then round down. (Example: 544 -> 500.)
  2. If the number in the tens column is 5 or more, then round up. (Example: 550 -> 600.)
Special Case:

If you get a number like 2950, it rounds up to 3000. Many students are confused by these kinds of problems.

Rounding to Nearest 100 and then Adding

Here's an example for you: 


      ORIGINAL    		ROUNDED
       PROBLEM			PROBLEM

       2344	---->		  2300
      + 566 	---->		 + 600
     -------		       --------
				  2900
In this example we first round each problem to the nearest 100 and then we add in the normal way. Rounding is very useful when estimating sums. Here's another example: 

      ORIGINAL    		ROUNDED
       PROBLEM			PROBLEM

       4550	---->		  4600
      +2922 	---->		 +2900
     -------		       --------
				  7500

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