Estimation is never an effective way to get the answer. But it's always an effective way to determine whether the answer you calculated is reasonable, or from orbit by way of left field.
Estimation is an effective way to determine an answer if we do not need an exact answer.
Explanation:
Estimation does not give us an exact answer. It gets us close to the exact answer, but it is not precise. This means we should use estimation to either check our work, or when we do not need a precise answer.
