Decimals – Multiplication and Division

These resources offer practice in operating on decimals and, moreover, opportunities to think about the processes of multiplication and division. As stated in the NCTM Focal Points, students should make sense of these procedures, become fluent in performing them, and be able to apply them in solving problems. I believe these sites, as a whole, offer practice in both understanding and problem solving.

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Learning about Multiplication Using Dynamic Sketches of an Area Model
In this applet, a rectangle represents the familiar area model of multiplication. By changing the height of the rectangle, students can explore the effect of multiplying a fixed positive number, in this case 3, by decimal numbers greater than 1 and less than 1. The visual is powerful!

Too Big or Too Small?
Scroll down to Activity 3: Exploring the Effect of Operations on Decimals. Through playing the cleverly crafted game presented here, students explore the effect of operations on decimal numbers. They begin with the number 100 as they enter a maze. For each segment chosen on the maze, the student calculates the assigned operation and number; for example, “+ 1.2” or “x 0.8.” The goal is to choose a path through the maze that results in the largest value at the finish.

Decimals
This site has explanatory lessons and interactive practice on most aspects of decimals, including multiplying decimals and dividing them. A good set of materials for self-tutoring or review. 

Find the Cost of Meat per Week at a Zoo
In a multi-step, NAEP assessment item, students must determine how much a zoo spends each week on meat to feed the animals. The site links to the scoring guide, sample student responses, and data on how well grade 8 students did on this multiplication/division problem. Only 13% solved it correctly!

Where’s the (Decimal) Point? asks students to explain clearly how they know where to put the decimal point in multiplication and in division of decimals. Students must think beyond the “rules” to the “whys.” I suggest these problems as challenges for older middle school students who are ready to stretch their thinking to the level of generalizing arithmetic.


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