Multiplying Tens

Using Equipment

Set up 4 x 20 as a model using the place value equipment you have chosen:

Ask the students to work out the total number in the collection. Discuss their strategies with a view to connecting 4 × 2 = 8 to 4 x 20 - 80.

Pose other similar problems modelling them each time with place value materials.
Examples could be:

3 x 30 = 3 × 3 = 9
2 × 40 = 80 from 2 × 4 = 8
6 × 50 = 300 from 6 × 5 = 30
5 × 30 = 150 from 5 × 3 = 15
Extend the problems to involve hundreds. Pose the problem 3 × 300.

Link the problem to 3 × 3 = 9.

Pose similar problems, such as:
4 × 200 = 800 from 4 × 2 = 8
2 × 300 = 600 from 2 × 3 = 6
Ask students to explain the pattern they use to solve these types of problems.
 

Using Imaging

Shielding: Require the students to image the problems by masking the canisters or stacks with icecream containers. Label each container with the number involved using post-its. Ask them to detail the structure of the numbers. For example, “Forty is four tens.”

What is 6 × 40?

Good examples to use are:

7 × 20, 8 × 50, 4 × 30, 7 × 500, 5 × 400, 6 × 500

Using Number Properties

Use calculators to explore more complex examples so that students can observe patterns in the answers. Connected examples might be:

7 × 50, 70 × 5, 500 × 7, 70 × 50, 500 × 70, 700 × 500

60 × 2 , 20 × 60, 200 × 6, 60 × 200, 600 × 200, 2000 × 60

Look for students to generalise the number patterns. Some will focus on the role of zero in describing how many digits the number will have. Develop estimation ideas through questions like, I have a two digit number multiplied by a three digit number. How many digits might the answer have? (four or five)