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

## Purpose

To practice mental multiplication by 10 and 100

None

To review mental multiplication, say a number that is a multiple of 10 (a “decade” number) and have a student multiply *that* number by 10. For example, if your number is 60, the student would say 600. Say numbers and call on students quickly to keep the activity fast-paced. Repeat the activity, asking students to multiply by 100. To keep the pace moving, select volunteers by using a deck of cards with their names (and don’t automatically shuffle the card to the back after using) to keep them engaged and ready to share.

Part 1 asks students to multiply decade numbers by 10. Part 2 has students multiply any two-digit numbers by 10 or 100. The extension asks students to multiply any number by either 10, 100, or 1,000.

Let’s see how many numbers we can multiply by 10. Here we go!

Examples:

- 20 × 10 (200)
- 40 × 10 (400)
- 30 × 10 (300)
- 60 × 10 (600)
- 90 × 10 (900)
- 80 × 10 (80)
- 40 × 10 (400)

Now let’s multiply any two-digit number by either 10 or 100. For example, if I say 27 × 10, your answer is 270. If I say 32 × 100 the answer is 3,200. Here we go!

Examples:

- 15 × 100 (1,500)
- 23 × 10 (230)
- 17 × 10 (170)
- 31 × 100 (3,100)
- 11 × 10 (110)
- 92 × 10 (920)
- 41 × 100 (4,100)
- 33 × 100 (3,300)

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

Now let’s multiply any number by 10, 100 *or* 1,000.

- 4 × 1,000 (4,000)
- 12 × 100 (1,200)
- 7 × 1,000 (7,000)
- 23 × 100 (2,300)
- 67 × 100 (6,700)
- 11 × 10 (110)
- 5,400 × 100 (540,000)
- 120 × 1,000 (120,000)
- 3,333 × 1,000 (3,333,000)

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