Mental Math: Halving & Doubling

By Noel McMillin

Mental Math Strategy:

Halving & Doubling

Multiplying with two digit numbers can be tricky!

• 12 x 15?

• 8 x 22?

• 14 x 25?

However, there is a trick to make a multiplication problem easier!

Halve one number and double the other number!​

For example, multiplying 12 x 15 is the same as multiplying 6 x 30.

• ​Half of 12 is 6.

• Double 15 is 30.

• 6 x 30 = 180 therefore, ​12 x 15 = 180

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Halve & Double or Double & Halve!

45 x 4

24 x 6

14 x 15

25 x 12

16 x 15

5 x 44

90 x 2 = 180

12 x 12 = 144

7 x 30 = 210

50 x 6 = 300

8 x 30 = 240

10 x 22 = 220

Multiple Choice

Halve & Double

15 x 24

(Choose the equivalent multiplication problem)

• 30 x 12

• 30 x 48

• 7.5 x 24

• 30 x 24

Multiple Choice

Halve & Double

12 x 25

(Choose the equivalent multiplication problem)

• 12 x 50

• 50 x 6

• 6 x 50

• 6 x 25

Multiple Choice

Halve & Double

15 x 16

(Choose the equivalent multiplication problem)

• 30 x 16

• 30 x 32

• 30 x 8

• 15 x 32

Multiple Choice

Halve & Double

24 x 5

(Choose the equivalent multiplication problem)

• 12 x 10

• 24 x 10

• 10 x 48

• 12 x 2.5

Suggestions to make it easier...

Never halve an odd number!

For example, 15 x 6 ​is easier if it is 30 x 3 rather than 7.5 x 12.

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Halve and double as much as you can!

For example, 36 x 8 ---> 72 x 4 ---> 144 x 2 ---> 288 x 1​​

Multiple Choice

Use the Halve & Double Strategy

25 x 8

• 50 x 4 = 200

• 25 x 16 = 400

• 50 x 8 = 400

• 12.5 x 4 = 50

Multiple Choice

Use the Halve & Double Strategy

16 x 15

• 16 x 30 = 480

• 32 x 15 = 480

• 8 x 30 = 240

• 16 x 30 = 240