Lesson 4

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Make sure you know you know your basic multiplications of one-digit number (like 8 x 7) with “Numbers Jugging - Times without the Tables.”

If you are driving, it will be hard to do and concentrate. You may want to listen if you have kids in the back of the car, and let them do the work. You should do the work later when you aren’t driving.

This lesson is about how to multiply four-digit numbers by 2. More than one of the digits will be greater than 5, making more than one carry.

An example would be 2 x 6,498.

Another would be 9537 x 2. (Please notice that there is no comma in this number. There is no law that says you have to use commas. We just use them as a convenience, so that it is easier to keep your place. Schools often insist that you use them, and penalize you if you don’t. That is silly, but we’ll probably always have to live with it, so it’s a good Idea to use them.)

For this lesson, we’re not going to use the memory technique we learned in the last lesson, because it gets a little cumbersome to use with more than 3 digits.

There are better memory techniques, but most of them are beyond the scope of this course. I will teach some, though, as we go along.

You still don’t have to use a pencil and paper to use this method with large numbers, though, as long as you can see the problem on paper, or on the blackboard. If, on the other hand, someone calls out the numbers, you’ll probably have to write them down.

We don’t have to write any “work,” or the carries, or the answer. If you can see the question, you can just start calling off the answer from left to right.

We’ll also be talking about different ways to look at multiplying the number 9,999 by two. How to double that? You can think of it as one group of 2 less than double 10,000. Double 10,000 is easy; it’s 20,000.
One group of 2 less than that means 20,000-2, which is 19,998.

Also, just by looking at a string of nines, we know that when we double all of these digits, they will all get a carry of 1, except for the final 9 (to which there is nothing to be carried from).

You may want to get out a deck of cards for this lesson. We are going to use them as a “worksheet.” Take out the jacks, queens and kings - we won’t be needing them for this. The tens will count as zeroes, the aces will count as ones. Now we can represent all ten digits with a deck of cards.

In the audio lesson, you’ll learn how to use them as unlimited “worksheets” Just take the deck out, shuffle it, and deal out the top four cards face up, and multiply that number by two.

Do that until you run out of cards in the deck. You’ll have done ten sets of four-digit numbers.

For a complete transcript of this podcast, click here.
(Right click on the link and and select from the menu if you want to download the PDF file to your computer.)

 

 Eating Math for Breakfast Audio Lesson 4 [9:21m]: Play Now | Play in Popup | Download