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Published byStephanie Barnett Modified over 8 years ago
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Mentally Multiply & Divide
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You already do mental math! 123 x 10 = 1,230 123 x 100 = 12,300 123 x 1000 = 123,000 How is it different to do… 12.3 x 10 = 123 12.3 x 100 = 1,230 12.3 x 1000 = 12,300
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All you do is move the decimal!
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Working with ‘Big’ Numbers 10, 100, 1000, 10000, … Multiplying : Move the decimal point RIGHT the same number of times as there are zeros in the number! Example: 12.045 x 100 = 1,204.5 Dividing : Move the decimal point LEFT the same number of times as there are zeros in the number! Example: 12.045 ÷ 100 = 0.12045
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Working with ‘Little’ Numbers 0.1, 0.01, 0.001, 0.0001 … Multiplying : Move the decimal point LEFT the same number of times as there are digits after the decimal point! Example: 12.045 x 0.0001 = 0.0012045 Dividing : Move the decimal point RIGHT the same number of times as there are digits after the decimal point! Example: 12.045 ÷ 0.01 = 1,204.5
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Remember: If you multiply by a BIG number, you move to the RIGHT. If you multiply by a LITTLE number, you move to the LEFT. When you divide, everything moves in the OPPOSITE direction.
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Practice 7,845.2 x 1000 = 7,845,200
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Practice 7,845.2 ÷ 1000 = 7.8452
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Practice 7,845.2 ÷ 0.001 = 7,845,200
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Practice 7,845.2 x 0.1 = 784.52
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Practice 0.203 ÷ 10 = 0.0203
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Practice 1900 x 1000 = 1,900,000
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Practice 700.15 x 0.0001 = 0.070015
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Practice 0.004 ÷ 0.01 = 0.4
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Practice 0.0003 x 1000 = 0.3
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Practice 19 x 0.0001 = 0.0019
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Practice 100 x 13.52 = 1,352
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Practice 9,912.78 ÷ 10,000 = 0.991278
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Exit Ticket 1. 17.5 x 0.001= 2. 10,000 x 0.45 = 3. 945 ÷ 0.01 = 4. 0.045 ÷ 10 = 5. 15.2 x 0.0001=
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