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 Skip Navigation LinksMath Help > Basic Math > Mental Math

It's important to know how to do math problems in your head, because that way you can double-check the results you get from your computer.  I was astonished the other day when one of my kids did a problem on the calculator: find the square root of 80, got an answer of 28.28, and just wrote it down and went on to the next problem.  "NO!"  I hollered.  "It must be less than NINE because the square root of 81 is NINE!"

Contents of this section:

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Mental Math and Estimating -- "Mental Math" is all about estimating.  It's also about clever tricks to get close to the right answer.  This is good not only for reasonableness checking, but also when close is good enough.

The Log Base 10 of a number is a good tool for estimating powers of 2, 3, 5, or even 7.  This page shows why it's smart to divide the scale of logs between 1 and 10 into 40ths, or even 568ths.

Related pages in this website

Word Problems, including percents, and the meaning of the words "of", "per", and "what", and much, much more.

Procedures

The Expansions page gives methods for calculating values, such as Gauss' method for calculating pi.

 

The webmaster and author of this Math Help site is Graeme McRae.