GoalUse basic facts to multiply and divide with multiples of 10, such as 3 × 70 and 240 ÷ 6.
New words
multiple of 10 · a number such as 20, 70 or 400 that has 0 in the ones placebasic fact · a multiplication fact from the tables, such as 3 × 7tens · groups of ten; 7 tens is 70
Explanation
To multiply by a multiple of 10, use a basic fact. To find 3 × 70, think of 70 as 7 tens. Then 3 × 7 tens = 21 tens, and 21 tens is 210. So 3 × 70 = 210. A quick way is to do 3 × 7 = 21 and write one zero after it. In the same way, 4 × 50 = 200 because 4 × 5 = 20, and 8 × 30 = 240 because 8 × 3 = 24. Division works backward. To find 240 ÷ 6, think of 240 as 24 tens. Then 24 tens ÷ 6 = 4 tens, which is 40. A missing factor works too: ? × 10 = 600 means 600 ÷ 10 = 60. Always check by multiplying back.
Fact
Think
Answer
3 × 10
3 tens
30
7 × 10
7 tens
70
12 × 10
12 tens
120
Worked examples
Find 3 × 70. Three groups of 70 are 3 × 7 tens = 21 tens, which is 210.
Basic fact
Tens fact
4 × 5 = 20
4 × 50 = 200
6 × 4 = 24
6 × 40 = 240
8 × 3 = 24
8 × 30 = 240
9 × 2 = 18
9 × 20 = 180
Every tens fact starts from a basic fact. Do the basic fact and write the zero: 4 × 5 = 20 so 4 × 50 = 200, and 8 × 3 = 24 so 8 × 30 = 240.
Find 240 ÷ 6. The basic fact is 24 ÷ 6 = 4, so 240 ÷ 6 = 40. The missing number in the fact triangle is 40. Check: 6 × 40 = 240.
Class activity
Tens fact race. Write ten basic facts on cards, such as 7 × 4 and 6 × 5. Turn each into a tens fact by multiplying one of the numbers by 10. Swap with a partner and solve. Then write three division facts with 100s, like 360 ÷ 9, and solve them using the basic fact.
Practice
1
Find 6 × 30.
6 × 3 = 18, so 6 × 30 = 180.
2
Find 8 × 50.
8 × 5 = 40, so 8 × 50 = 400.
3
Find 420 ÷ 7.
42 ÷ 7 = 6, so 420 ÷ 7 = 60. Check: 7 × 60 = 420.
4
Find the missing number: ? × 10 = 900.
900 ÷ 10 = 90, so the missing number is 90.
5
A class trip costs 40 LE for each child. How much do 9 children pay altogether?
