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September 21, 2014

September 21, 2014

Posted by **B.B.** on Tuesday, June 9, 2009 at 7:55pm.

- Math -
**MathMate**, Tuesday, June 9, 2009 at 10:51pmYes, your method satisfies what the teacher needs, namely give an estimate of the value, and be able to state whether the exact answer is higher or lower than your estimate.

If you want to have a closer estimate (or even the exact answer), here's what I would do, knowing that you already know how to multiply a number by 11 mentally.

54*23

=55*23 - 1*23

=11*(23*5) - 23

=11*115 - 23

=1265 -23

1265 is the estimate, it is an over estimate.

The exact answer can also be calculated mentally as 1265-23=1242

Note: To multiply a number by 5, add a zero and divide by 2.

23*5 = 230/2 =115

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