We're thinking of decimal multiplication conceptually in this video, just working towards the common shortcut (which I introduce in part 2 of this lesson, found at https://www.youtube.com/watch?v=IvQIK5q6x1g )

We start out with problems that have a decimal times a whole number. For example, we can see 0.4 x 20 as 4/10 of 20 -- the multiplication here "translates" or corresponds to the word "of", or to the idea of taking a fractional part of the quantity. To figure that out, first find 1/10 of 20, which is 2. Then, 4/10 of 20 is four times that, or 8.

After practicing with few decimal multiplication problems along those lines, we go on to seeing decimal multiplication as SCALING.

I use the visual model of a stick, which we "scale", or make longer or shorter, by using decimal multiplication. We still first find 1/10 of the quantity, and based on that, solve the multiplications, such as 0.7 x 20 or others. But soon it is time for the SHORTCUT.

Tagged under: decimal multiplication,scaling, number,decimal, number times decimal,multiplication,math,grade 5

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