topic badge

6.05 Multiply decimals by 2 digit numbers

Lesson

Are you ready?

Do you remember how to use a standard algorithm to multiply decimal numbers? Let's try this problem to practice.

Find $10.48\times6$10.48×6, giving your answer as a decimal.

Vocabulary:
  • You might notice that sometimes the standard algorithm is called the 'vertical algorithm'. Let's think about why. When we use the standard algorithm, we line our numbers up in 'vertical' place value columns.

Learn

This video looks at how to use the area model to multiply numbers with decimals. 

Apply

Question 1

Use the area model to find $46\times4.8$46×4.8.

  1. Fill in the areas of each rectangle.

    $4$4 $0.8$0.8
    $40$40 $\editable{}$ $\editable{}$
    $6$6 $\editable{}$ $\editable{}$
  2. What is the total area of the four rectangles, and therefore the answer to $46\times4.8$46×4.8?

 

Learn

This video shows how to multiply a decimal number in the hundredths by a $2$2 digit number using the standard algorithm.

Apply

Question 2

We want to find $43\times1.07$43×1.07.

  1. First, choose the most reasonable estimate for $43\times1.07$43×1.07.

    $23$23

    A

    $92$92

    B

    $43$43

    C
  2. Find $43\times1.07$43×1.07, giving your answer as a decimal

 

Remember!

When using the standard algorithm to multiply a decimal number by a two digit number we find the product of the first and one of the digits in the second row. After we do this for each digit in the second row, we add all the products together.

 

Outcomes

5.NBT.B.7

Add, subtract, multiply, and divide decimals to hundredths, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between operations. Assess the reasonableness of answers using estimation strategies. (Limit multiplication problems so that the product does not exceed thousandths. Limit division problems so that either the dividend or the divisor is a whole number.)

What is Mathspace

About Mathspace