Grade: 5, Topic(s): multiplication/division

## Purpose

To multiply two multiples of 10 to get a multiple of 100

None

Depending on your class, you might begin asking what happens when you multiply two multiples of 10 (a multiple of 100). Keeping a quick and lively pace, give students mental math practice multiplying multiples of 10 such as 70 x 80. Occasionally ask students to reply by stating the product as hundreds (fifty-six hundreds) and as thousands and hundreds (five thousand, six hundred). Ask occasionally, “How many hundreds are in 5,600?” Some may quickly reply “6” even though there are actually 56.

Part 1 asks students to mentally multiply two multiples of 10 to get a product that is a multiple of 100. Start with 2-digit decade numbers. Part 2 continues multiplying two multiples of 10 but ask students to restate the product as hundreds (for example, 60 x 30 can be restated as eighteen hundreds). In the extension, children multiply multiples of 10 by hundreds and identify the number of hundreds in each of these larger products.

If 2 × 10 = 20, what to do you think 20 × 30 equals? (At this point, students may add 2 zeros to the product of 2 × 6 which equals 600.) Let’s multiply some more multiples of 10.

Examples:

- 10 × 10 (100)
- 20 × 10 (200)
- 40 × 10 (400)
- 30 × 10 (300)
- 20 × 30 (600)
- 40 × 20 (800)

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

Let’s keep multiplying two multiples of 10 but now there are 2 parts to each answer. 1) Name the product. 2) How many *hundreds* are in the product? We’ll do the first few together!

Examples:

- 20 × 30 (600; 6 hundreds are in 600)
- 30 × 50 (1,500; 15 hundreds)
- 20 × 60 (1,200; 12 hundreds)
- 40 × 30 (1,600; 16 hundreds)
- 60 × 30 (1,800; 18 hundreds)
- 50 × 70 (3,500; 35 hundreds)
- 60 × 40 (2,400; 24 hundreds)

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

Now let’s multiply multiples of 10 by hundreds. How many *hundreds* are in the product?

Examples:

- 20 × 300 (6,000; 60 hundreds are in 6,000)
- 40 × 700 (28,000; 280 hundreds are in 28,000)
- 90 × 600 (54,000; 540 hundreds are in 54,000)

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