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July 24, 2014

July 24, 2014

Posted by **B.B.** on Monday, June 8, 2009 at 7:36pm.

- Math -
**MathMate**, Monday, June 8, 2009 at 11:15pm980*70

=(978+2)*(66+4)

=978*66 + (2*66 + 4*978 + 4*2)

Since the terms in parentheses are all positive, so the estimate must be more than the correct answer.

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# following text for mental calculation

# enthusiasts only

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In fact, you can actually*calculate*the exact value mentally, if you are prepared to exercise your grey matters a little more than usual. Proceed as follows:

978*66

=(1000-22)*66

=1000*66 - 22*66

=66000 - 2*66*11

=66000 - 132*11

=66000 -1452

=64548

When a number is multiplied by 11, you only have to add the adjacent digits to make a new digit. Assume you have a leading and a trailing zero. For example,

12*11

= 0+1 || 1+2 || 2+0

= 132

22*66 can be calculated as follows:

=(2*6)*11*11

= 132*11

= 0+1 || 1+3 || 3+2 || 2+0

= 1 || 4 || 5 || 2

= 1452

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