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Join NUMBEROCK under the sea and learn how to multiply by a decimal number using number sense to determine where the decimal point belongs in the product. Then take a peak above the sea where a pirate ship is battling it’s way through a terrifying lightning storm as we review the traditional method of placing the decimal point when multiplying decimals!
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Multiplying Decimals Lyrics:
Let’s use our number sense
to multiply nine and one hundredth
by five and one tenth.
We multiply the one to get one, zero, nine.
In the next place value there’s a five.
X holds the place on the second line;
we get a five, a zero, and a forty-five.
Write a line, a plus sign, tally up the score -
to get one-five-nine-five-four.
Let’s think about the reasonableness of our answer
by rounding to a simpler number.
Then we can easily see
where the point needs to be.
Now let’s get intense,
and multiply ninety-two hundredths
by three and two tenths.
Multiply the two, get four and eighteen;
in the next place value is a three.
Write an x to hold the place, and then
we get a six and a twenty-seven.
Add it up and get four, four,
a nine and a two, and there’s one step more.
Here’s another way to place the point worth trying:
look at the numbers you are multiplying,
count the number of decimal places.
In the product move left that many spaces.