MULTIPLIPLY, THEN
SUBTRACT.
When multiplying by 9, multiply by 10 instead, and then subtract the other number. For example,
23×9 = 230 - 23 = 207.
The same applies to other numbers near those for which
multiplication is simplified:
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23×51
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= 23×50 + 23
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= 2300/2 + 23
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= 1150 + 23
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= 1173.
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87×48
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= 87×50 - 87×2
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= 8700/2 - 160 - 14
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= 4350 - 160 - 14
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= 4190 - 14
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= 4176.
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