secrets-of-mental-math-malestrom

112
Secrets of Mental Math
8,367
5,819
2,548
8,000
ª 6,000
or
2,000
8,400
5,800
2,600
You can see that rounding to the nearest thousand leaves
you with an answer quite a bit off the mark. By rounding to the
second digit (hundreds, in the example), your answer will usually
be within 3% of the exact answer. For this problem, your
answer is off by only 52, a relative error of 2%. If you round to
the third digit, the relative error will usually be below 1%. For
instance:
439,412
24,926
414,486
440,000
ª
20,000 or
420,000
439,000
25,000
414,000
By rounding the numbers to the third digit rather than to the
second digit, you improve the accuracy of the estimate by a significant
amount.
DIVISION GUESSTIMATION
The first, and most important, step in guesstimating the answer to
a division problem is to determine the magnitude of the answer:
9,644.5
657,867.0
ª
9
658,000
54
4
Answer ª 9 2 thousand 9,667
3