Friday, June 21, 2013

JJ's New Math Skill--Carry Over Subtraction

Recently I got a set of wooden number rods (similar to the Montessori Number Rods, only in different colors and fit in a child's palm) at a sale for $1.  I write numbers on them to identify their lengths.  Then, I use them to introduce the concept of carry over subtraction to JJ.  Here's how:
  1. Set the minuend (the number that is to be subtracted from) using number rods.  In this case, 15--1 ten rod and 1 five rod.
  2. Since the subtrahend (the number that is to be subtracted) is greater than the one's place of the minuend, JJ needs to substitute the ten rod with 10 ones cubes. 
  3. Remove the ten rod.
  4. From the 10 ones cubes, subtract the subtrahend.  In this case, 6 ones cubes.
  5. Remove the cubes and get the difference.  Start by saying the name of the number rod (5), then add the remaining ones cubes to that number (6, 7, 8, 9).  Easy?  JJ thinks so, too.

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