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Level D Lesson 83 Multiplying 3 numbers

I had no problem teaching the beginning of this lesson to my son and he started worksheet #84 but the last 8 problems on the sheet gave both of us trouble. I had the hardest time trying to explain how these problems were like the rest of the worksheet. What I ultimately did was teach him the written method that I learned in school and moving on. Is there something I missed in how to teach this lesson? It seemed like a big jump from 6x25x4 to 795x80.

Re: Level D Lesson 83 Multiplying 3 numbers

Hi, Heidi.

Great question!

Take a look at the equations in the middle of the worksheet, namely, 2 x 10 x 5, 12 x 4 x 10, 13 x 10 x 2, 6 x 10 x 8 and 7 x 6 x 100. The concept is being 'pulled' out in these equations. For example, isn't 2 x 10 x 5 the same as 2 x 50? Or how about, 13 x 10 x 2. That is the same as 13 x 20. Or 7 x 6 x 100 is the same as 7 x 600. When applied to the equations at the bottom of Worksheet 84, take 68 x 20, the equation is 'thought of' as 68 x 2 x 10. The same thing goes for the rest of them:

17 x 5 x 10
215 x 3 x 10
32 x 4 x 10
156 x 3 x 10
948 x 7 x 10
795 x 8 x 10

By doing these exercises, they understand why when the multiply anything by '10' or '100' they basically add a '0' (or '00') to the end of the equation.

Does that make sense? Basically, the student is learning the 'why' of the algorithm prior to using the algorithm. When I was taught in school, I was told that whenever you see a '0' (or 2) at the end of one of the numbers in a multiplication equation, you simply need to multiply all the 'other numbers' and add a '0'. This was a process/procedure I was taught to memorize, but didn't fully understand 'why' we did this. That is the point of this lesson.

I hope that helps! If you have any further questions, please do not hesitate to repost here or email RightStart Math directly at info@rightstartmath.com.

Have a great day!
Rachel