1. Mental Methods
Reference Material
Text book: section 8.4
MEP:
use partitioning to multiply numbers mentally 5a
use factors to multiply mentally
multiply mentally using doubling and halving
multiply mentally
solve problems mentally 6c
use doubling and halving to multiply negative numbers
multiply any number by 0.1 and 0.01 6b
divide any number by 0.1 and 0.01 6a

2. use factors to multiply mentally 5a
53 x 20 = 53 x 10 x 2 = 530 x 2 = 1060
1.5 x 300 = 1.5 x 100 x 3 = 150 x 3 = 600
15 x 2.8 = 15 x 2 x 0.8 = 30 x 0.8 = 30 ÷ 10 x 8 = 3 x 8 = 24
multiply mentally using doubling and halving 5a
4.8 x 50 = 4.8 x 10 x 5 = 48 x 5 = 24 x 10 = 240
6.8 x 2.5 = 3.4 x 5 = 1.7 x 10 = 17
3.8 x 3.5 = 1.9 x 7 = 1 x x 7 = = 13.3
10 September 2019

3. Your Turn
12 x 1.5
8.3 x 20
8.5 x 25
6.3 x 21
7.5 x 16
14 x 3
3.5 x 6.4
5.6 x 2.5
5.5 x 8
7.5 x 6
= (12 x 1) + (12 x 0.5) = = 18
= 8.3 x 2 x 10 = 83 x 2 = 166
= 85 x 2.5 = 850 x 0.25 = ¼ of 850 = ½ of 425 = 212.5
= 6.3 x x 1 = = 132.3
= 15 x 8 = 30 x 4 = 120
= 7 x 6 = 42
= 7 x 3.4 = 7 x x 0.4 = = 23.8
= 56 x 0.25 = ¼ of 56 = ½ of 28 = 14
= 11 x 4 = 44
= 15 x 3 = 45
10 September 2019

4. multiply any number by 0.1 and 0.01 6b
Multiplying by 0.1 is the same as multiply by which is the same as ÷ 10
Multiplying by 0.01 is the same as multiply by which is the same as ÷ 100
Any number means decimals and negative numbers as well
10 September 2019

5. 5.2 x 0.1
-76.3 x 0.1
-45.9 x 0.01
0.81 x 0.01
791.4 x 0.01
x 0.1
0.79 x 0.1
412.1 x 0.01
-1.23 x 0.1
-3.08 x 0.01
= 5.2 ÷ 10 = 0.52
= ÷ 10 =
= ÷ 100 = -0.45
= 0.81 ÷ 100 =
= ÷ 100 = 7.914
= ÷ 10 =
= 0.79 ÷ 10 = 0.079
= ÷ 100 = 4.211
= ÷ 10 =
= ÷ 100 =
10 September 2019