Grade: 4, Topic(s): multiplication/division

## Purpose

To multiply numbers by multiples of 10

None

To practice mental computation, have students multiply one-digit numbers by 10, 100, 20, 30, and 40. Keep the activity moving quickly so that all students have a chance to participate. If necessary, you might remind students that 20 is 2 x 10, 30 is 3 x 10, and 40 is 4 x 10. Students may find that breaking up 20, 30, and 40 in this way is easier than multiplying by those numbers directly.

Part 1 asks students to multiply one-digit numbers by 10, 100 and 20. Part 2 asks students to multiply one-digit numbers by 30 and 40 (in addition to multiplying by 10, 100 and 20). The extension multiplies two-digit numbers by 10, 100, 20, 30 and 40.

Let’s multiply one-digit numbers by 10, 100 and 20. (If multiplying by 20 feels challenging, remember 20 is the same as 2 × 10.)

Examples:

- 3 × 10 (30)
- 3 × 100 (300)
- 3 × 20 (60)
- 2 × 10 (200)
- 2 × 100 (200)
- 2 × 20 (40)
- 5 × 100 (500)
- 5 × 10 (50)
- 5 × 20 (100)
- 7 × 100 (700)
- 7 × 10 (70)
- 7 × 20 (140)

*While children are enjoying their building of mastery, feel free to repeat. When children are eager for more, try Part 2.*

Let’s multiply one-digit numbers by 30 and 40, as well as by 10, 100, 20. (Remember multiplying by 30 is 3 × 10 and 40 is 4 × 10). Here we go!

Examples:

- 4 × 20 (80)
- 4 × 30 (120)
- 4 × 40 (160)
- 4 × 100 (400)
- 6 × 20 (120)
- 6 × 30 (180)
- 6 × 40 (240)

*As always, when children seem excited for a new challenge, move on.*

Let’s multiply any two-digit numbers by 10, 20, 30, 40, and 100.

Examples:

- 20 × 30 (600)
- 60 × 40 (2,400)
- 70 × 20 (1,400)
- 90 × 40 (3,600)
- 40 × 100 (4,000)
- 50 × 10 (500)
- 33 × 30 (990)
- 87 × 100 (8,700)

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