Mental Math

Mental Math

"do math in your head"

​Strategies

• Estimation of products

• ​Multiplying by multiples of 10, 100, and 1, 000

• ​half of 10 strategy

• ​compensation strategy

• ​partitioning strategy

​Estimate the product of 124 and 8.

​First, round off the number to their highest place value. Then, mulitply.

​

​100 x10 = 1, 000

​128 x 4 = N

​124

​

​x 8

1,092​

The actual product of 124 and 8 is 1, 092. Thus, our estimation is correct.

​Estimate the product of 5, 267 and 75.

​First, round off the number to their highest place value. Then, mulitply.

​

5, 000 x 80 = 400, 000

​5, 267 x 75 = N

5, 267​

​x 75

26, 335

• + 386, 690

• 395, 025

The actual product of5, 267 and 75 is 395, 025. Therefore, our estimation is correct.

Multiple Choice

Which is the correct estimation for the number sentence " 7, 294 x 19 = N"?

• 140, 000

• 14, 000

• 1, 400

• 140

Multiple Choice

Which is the correct estimation for the number sentence " 175 x 9 = N"?

• 200

• 2, 000

• 20

• 20, 000

​Multiples of 10, 100, and 1, 000

A number is a multiple of 10 if it has zero at the ones place value.

​Ex.: 20, 40, 110

A number is a multiple of 100 if it has zero at tens and ones place value.

​Ex.: 200, 500, 1, 200

A number is a multiple of 1, 000 if it has zero at the hundreds, tens, and ones place value.

​Ex.: 12, 000, 876, 000

​A shortcut in multiplying numbers by multiples of 10

​1, 345 x 10 = N

Multiply 1, 345 by 1 then write 0 at the end of the product.

13450 or 13, 450

​1, 345 x 10 = 13450 or 13, 450

​1, 345 x 100 = 134500 or 134, 500

​1, 345 x 1, 000 = 1345000 or 1, 345, 000

Look at the pattern

12 x 40 = 480

12 x 400 = 4800 or 4, 800

​12 x 4, 000 = 48000 or 48, 000

Look at the pattern

Fill in the Blanks

12 x 10 =

Fill in the Blanks

5 x 500 =

Fill in the Blanks

3, 000 x 6 =

​Half of 10 Strategy

​This strategy is best use in multiplying numbers by 5. Basically, the half of 10 is 5. So, we can get the half of the number and multiply it by 10.

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​Half of 10 Strategy

​This strategy is best use in multiplying numbers by 5. Basically, the half of 10 is 5. So, we can get the half of the number and multiply it by 10.

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