Mental Math

• Estimation of products

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

• ​half of 10 strategy

• ​compensation strategy

• ​partitioning strategy

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

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​100 x10 = 1, 000

​124

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​x 8

1,092​

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

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

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5, 000 x 80 = 400, 000

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.

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

​1, 345 x 10 = N

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

12 x 40 = 480

12 x 400 = 4800 or 4, 800

​12 x 4, 000 = 48000 or 48, 000

​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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​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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