Explanation:
If your actual answer is very far from your estimate, you probably made a mistake somewhere.
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<em>Additional comment</em>
50 years ago, when a slide rule was the only available calculation tool, making an estimate of the result was a required part of doing the calculation. Not only were the first one or two significant digits needed, but also the power of 10 that multiplied them. Use of a slide rule required the order of magnitude be computed separately (by hand) from the significant digits of the result.
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You may also find it useful to estimate the error in your estimate. That is, you may want to know the approximate answer to 2 (or more) significant digits in order to gain confidence that your calculation is correct.
