You will use a variety of estimating strategies on the GRE. One important strategy for estimating with fractions is to use Benchmark Values. These are simple fractions with which you are already familiar:
You can use Benchmark Values to compare fractions:
Which is greater:
?
If you recognize that 127 is less than half of 255, and 162 is more than half of 320, you will save yourself a lot of cumbersome computation.
You can also use Benchmark Values to estimate computations involving fractions:
Approximately what is
of 2,000?
If you recognize that these fractions are very close to the Benchmark Values
and
, you can estimate:
Notice that the rounding errors compensated for each other:
You decreased the denominator, so you rounded up:
.
You increased the denominator, so you rounded down:
.
Note also that
so your estimation was very close.
If instead you had rounded
instead, then you would have rounded both fractions up. This would lead to a slight but systematic overestimation:
Try to make your rounding errors partially cancel each other out by rounding some numbers up and others down.
Which is greater:
?
