Estimation gives you an approximate result before or alongside exact calculation. It is useful for checking scale, eliminating impossible options and noticing a misplaced decimal point.
Round with a purpose
For 19.8 × 5.1, using 20 × 5 gives about 100. The exact product is 100.98. An option near ten or one thousand can be rejected by scale alone.
The estimate is not the exact answer. If options are 100.8 and 100.98, you need a more precise calculation. Match the precision of your method to the difference between the options.
Use upper and lower bounds
If a positive quantity lies between 20 and 21, then multiplying it by four places the result between 80 and 84. A candidate answer of 90 cannot fit.
Bounds can be more reliable than a single rounded guess because they state a permitted interval. Remember that multiplying inequalities by a negative number reverses their direction; basic positive-quantity problems avoid that extra complication.
Compare with familiar fractions
To estimate 31% of 198, notice that 30% of 200 is 60. The exact answer should be close to that. Calculating 0.31 × 198 gives 61.38.
If you obtained 613.8, the estimate immediately reveals a scale error. It does not tell you every arithmetic step that went wrong, but it tells you to check.
Avoid rounding every intermediate step
Repeated early rounding can accumulate error. For an exact result, keep sufficient precision until the end. Use the rough estimate as a parallel check rather than replacing every value in the detailed calculation.
A useful practice habit
Before solving a numerical question, predict a range and a unit. After solving, compare the result with both. A number may have the right size but the wrong meaning—for example, a percentage increase instead of a final percentage—so estimation should complement careful reading, not substitute for it.