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Constructing counting on to solve subtraction in the drawing problem

12 April 2011

Counting on, commonly used to deal with situation related to addition, is a handy strategy to cope with some subtraction problems. When children having constructed this kind of strategy are adding 8 + 4, rather than counting from one, they start with 8 and say “9, 10, 11, 12”. Teacher should arrange learning environment that can support students constructing counting on strategy when working with subtraction problem. It can be done by promoting a right context, a situation intended to develop mathematizing, suggesting counting on strategy such as this drawing problem.

“Isa has drawing as her hobby. She has a lot of drawing books full of fascinating drawings in her room. Right now, she wants to draw a lot of flowers. Can you help her to solve this problem? Isa wants to draw 16 flowers. How many flowers does she need to draw more if she has already drawn 9 flowers?”

Using this subtraction context, at the first, most students will draw flowers one by one while counting. When drawing, they will construct an understanding that 9 comes before 16 or 9 is included in 16, so they need to count how many flowers after the 9th flower to get into the 16th one.  Hence, they will grasp the idea that they do not need the first nine flower and only need to count on 10, 11, 12, 13, 14, 15, and 16 to come up with 7 as the solution.

Using drawing problem as a subtraction context, teacher does not need to tell student how to do it since most students love drawing. He only needs to give the situation that can help students to construct the counting on strategy, and involving drawing is a good way to develop students’ understanding especially for early grades. (ham)

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