Strategies Used For Mental

Multiplication/Division of Larger Numbers

June 4th 2008

Year 5/6NH

Strategy 1

Use or relate a known fact 4x15: I know that 3x30 =90 so 4x15

must be same as 2x30,

7x25 : I know that 4 x 25 = 100 so 7 x 25 must be 100 + 75 = 175

Try this3x16:

Strategy 2Use extension of 1 digit multiplication strategies:

eg. Multiple Strategy Example 2x Double 2 x 24 : Double 24=48 3x Double and 1 more 3 x 24 : Double 24+24=72 4x Double twice 4 x 24 : 2 x 24 = 48, 2x48=96 5x Half of 10 times 5 x 24 : 10 x 24 = 240, half of

240=120 6x Five times and one more 6 x 24 : 120 + 24 = 144 7x Five times and one more 7 x 24 : 120 + 48 = 168 8x Double 3 times 8 x 24 : 48, 96, 192 9x One less than 10 times 9 x 24 : 240 -24 = 216

Try this 8 x 13

Strategy 3. Skip Count

There are a few two-digit numbers for which we may be able to use skip counting

4 x 30:

30, 60, 90, 120

5 x 21:

21, 42, 63, 84, 105

Try this:

6 x 15

Strategy 4:Strategy 4:Using the Distributive PropertyUsing the Distributive Property

This is the mental equivalent to the normal written algorithm, This is the mental equivalent to the normal written algorithm, in which we multiply separately the tens and the ones unit in which we multiply separately the tens and the ones unit and then add the two results together.and then add the two results together.

eg : 4 x 27eg : 4 x 27

(4 x 20 ) + ( 4 x 7 )(4 x 20 ) + ( 4 x 7 )

80 + 28 80 + 28

108108

Try this:Try this:

8 x 178 x 17

Strategy 5: Using Multiplication To Help Solve Division

ProblemsMost of us are more comfortable with multiplication that with

division. Therefore, we often solve a division by turning it into multiplication.

eg : 68 divided by 4 : can become

4 x ? = 68

We know that 4 x 10 = 40, that leaves 28.

So 4 x 7 = 28, so:

4 x 17 = 68 and 68 /4 = 17

Try this:

108 divided by 4

Strategy 6:

Dividing Using the Distributive Method

Here we split the number we are dividing into two convenient parts then dividing each by the divisor. The

answers are then added together.

eg . 78 divided by 6

We spilt 78 into

60 + 18

Then 60 divided by 6 = 10

And 18 divided by 6 = 3

Add the answers 10 + 3 = 13

Try this:

192 divided by 8