3
Multiplying multiples of 10
Discover Mathematics P3: Term 1, Lessons 41–50
GoalMultiply a one-digit number by a multiple of 10 using the basic fact and tens.
New words
multiple of 10 · a number made of whole tens, like 30, 70 or 90basic fact · a multiplication fact with small factors, like 3 × 7pattern · something that repeats in a way you can predict
Explanation
A multiple of 10, such as 70, is 7 tens. To find 3 × 70, think of 3 groups of 7 tens. 3 × 7 = 21, so 3 × 7 tens = 21 tens = 210. We write it as (3 × 7) × 10 = 21 × 10 = 210. First multiply the basic fact, then multiply by 10. For example, 8 × 40 = (8 × 4) × 10 = 32 × 10 = 320. Look at the pattern: the answer to 3 × 70 is the answer to 3 × 7 with one zero added at the end.
| Basic fact | With tens |
|---|---|
| 3 × 7 = 21 | 3 × 70 = 210 |
| 4 × 5 = 20 | 4 × 50 = 200 |
| 6 × 8 = 48 | 6 × 80 = 480 |
Worked examples
3 × 70. The tape shows 3 parts of 70. 3 × 7 = 21, so 3 × 70 = 210.
8 × 40. The tape shows 8 parts of 40. 8 × 4 = 32, so 8 × 40 = 320.
| Basic fact | With tens |
|---|---|
| 8 × 5 = 40 | 8 × 50 = 400 |
Class activity
Tens bundles. Use 60 craft sticks tied in 6 bundles of 10. Show 4 × 30 as 4 groups of 3 bundles, count the bundles and then the sticks. Do the same for 5 × 40 and 3 × 60. Write the basic fact and the answer each time.
Practice
1
Find 5 × 60. The tape shows 5 parts of 60.
5 × 6 = 30, so 5 × 60 = 300.
2
Find 4 × 30.
4 × 3 = 12, so 4 × 30 = 120.
3
Find 7 × 20.
7 × 2 = 14, so 7 × 20 = 140.
4
A box holds 40 pencils. How many pencils are in 6 boxes?
6 × 40 = (6 × 4) × 10 = 24 × 10 = 240 pencils.
5
Hana says 4 × 50 = 20, because 4 × 5 = 20. What did she forget? What is the right answer?
She forgot to multiply by 10. 4 × 50 = (4 × 5) × 10 = 20 × 10 = 200.