There are a variety of strategies that people use for developing this ability. For instance, while adding 1821,695 and 250, a person could estimate it mentally

i) by rounding off the numbers and computing with these numbers:

1800+700+200 or1800+700+300

ii) by considering the leftmost digits to form a rough estimate, and then adjusting this estimate by considering the other digits:

first 1000 + 660 + 200, and then add 800 + 100 + 50.

There are several other ways like clustering, combining compatible numbers, and so on Which stately is used depends on the problem under consideration. These, and other strategies, can only be developed by careful instruction, discussion and practice. Consider the following situation, related to this issue.