1. ©2010 G Dear – Not to be sold/Free to use
Number
Mental Arithmetic
Stage 5.1
Year 10
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©2010 G Dear – Not to be sold/Free to use

2. Mental Arithmetic – ‘+’
Evaluate the easy way.
Think 50 + (60 + 1)
then ( ) + 1)
equal
or 111

3. Mental Arithmetic – ‘+’
Evaluate the easy way.
Think 20 + (70 + 3)
then ( ) + 3)
equal
or 93

4. Mental Arithmetic – ‘-’
Evaluate the easy way.
Think (240 – 30) - 1
then
equal 209

5. Mental Arithmetic – ‘+’
Evaluate the easy way.
Think (260 – 2) - 30
then (260 – 30) - 2
then
equals 228
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6. Mental Arithmetic – ‘x ÷’
Numbers can be multiplied in any order.
When numbers are multiplied or divided
they may be split. [Factorized]
x 2
Double
÷ 2
Halve
x 4
Double twice
÷ 4
Halve twice
x 5
Multiply by 10 and then halve.
÷ 5
Divide by 10 and then double.

7. Mental Arithmetic – ‘x ÷’
Multiply by 10 and then double.
÷ 20
Divide by 10 and then halve.
x 25
Multiply by 100 and halve twice.
÷ 25
Divide by 100 and double twice.
x 50
Halve and then multiply by 100.
÷ 50
Double and then divide by 100.

8. Mental Arithmetic – ‘x ÷’
Multiply by (10 + 1).
÷ 101
Multiply by ( )
x 9
Multiply by (10 - 1).
÷ 99
Multiply by ( ) F1
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9. Mental Arithmetic – ‘x’
Evaluate 50 x 61 the easy way.
Think 100 ÷ 2 x 61
Think 100 x 61÷ 2
then 6100 ÷ 2
equal 3050

10. Mental Arithmetic – ‘x’
Evaluate 101 x 54 the easy way.
Think ( ) x 54
then 54 x ( )
then 54 x x 1
then
equals 5454

11. Mental Arithmetic – ‘x’
Evaluate 21 x 99 the easy way.
Think 21 x ( )
then 21 x x 1
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then
then 2100 –
then
equals 2079
Keyboard
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