Q&A #16511

Modeling Subtraction of Integers using Red and Yellow Counters

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From: Mimi <mimisolis@comcast.net>
To: Teacher2Teacher Service
Date: Jan 14, 2006 at 03:19:12
Subject: Modeling Subtraction of Integers using Red and Yellow Counters

Modeling -7 - 5 using red(-) and yellow(+) counters is a difficult concept to instruct. How can this best be described so that students can fully understand, therefore, accept the answer as -12 and not -2, -7 or some other integer? Students understand that they are subtracting 5 positive counters from 7 negative counters, but they don't want to accept -- for they don't understand -- that they have to add both red and yellow counters to the model so that it doesn't change the value of the number they are subtracting (in this case -7). My model would show 7 red counters and then 5 red and 5 yellow counters to illustrate that the value of -7 remains the same. When I explain this, I get perplexing facial expressions. They simply want to show 7 negative counters and 5 positive counters. They understand the concept of additive inverse, but they don't know that they need to apply it when subtracting a positive integer from a negative one. They really don't understand the idea of why the value of -7 in this problem must remain the same. They just don't understand what I am talking about here. How can I better describe this when modeling subtraction of integers using red and positive counters so that they understand that they are subtracting a positive number from a negative number? Thank you for helping! Mimi

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