Multiplication - The Four Operations On Decimals printable pdf download

Unit 7 - The Four Operations on Decimals

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Chapter 2 – Multiplication

Objectives

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Multiply decimal numbers by a 1-digit whole number.

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Use estimation to check reasonableness of answers.

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Solve word problems involving multiplication of decimal numbers.

Material

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Place-value discs (0.01, 0.1, 1, 10, and 100)

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Place-value chart

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Mental Math 15-16 (appendix pp. a5-a6)

Notes

In Primary Mathematics 2A, students learned the standard algorithm for multiplying a whole

number by a 1-digit whole number. In Primary Mathematics 3B, they learned to multiply money in

dollars and cents by a 1-digit whole number by converting the money to cents, multiplying, and

converting back to dollars and cents.

In this chapter, the standard algorithm will be extended to 1-place and 2-place decimals.

When writing addition or subtraction problems in vertical format, it is

6 . 1 4

important to always align the digits according to their place value. In

x 3

multiplication, however, we do not align digits for the two factors, since we

1 8 . 4 2

need to multiply the single digit with the value in each place value. While it is

possible to represent 6.14 x 3 in either of the two ways shown on the right,

6 . 1 4

only the second way will be used. In Primary Mathematics 5A, students will

learn how to multiply a decimal by a whole number greater than 10 or another

1 8 . 4 2

decimal, without regard to the decimal point when performing the actual

multiplication, and then place the decimal point correctly in the final answer.

As with addition and subtraction, always use the place values when discussing the process. 0.6 x

5 is “six tenths times five equals 30 tenths or 3 ones,” not “six times five is three.”

Students will be asked to estimate their answer in order to check if their answer is reasonable.

For multiplication, your student can round the decimal to one non-zero digit, e.g. 27.9 x 3 can be

estimated by using 30 x 3 = 90 and 0.279 x 3 can be estimated using 0.3 x 3 = 0.9.

In this chapter, your student will be solving 2-step word problems that involve multiplying

decimals. In Primary Mathematics 3A students learned how to use the part-whole and comparison

models to solve word problems involving multiplication. The drawing in Task 18 on p. 55 is an

example of a part-whole model, and the one in Task 19 on the same page is an example of a

comparison model. Modeling is used, but not re-taught here. Use the models during lessons, and

have your student draw them when doing the tasks, but use your discretion in requiring them for

independent work. Do not insist your student follow a prescribed set of steps in drawing the

models, since unless they learn to solve these problems logically and develop problem-solving skills

that don’t rely on following a set of steps, they will have difficulty in Primary Mathematics 5A.

Multiplication - The Four Operations On Decimals

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