Theory:

Suppose you are going to a stationery store. You brought $$₹100$$ to the shop. The following are the price of the things that you have taken in your cart.

 Item Price Colour book $$₹48$$ Colour pencil $$₹23$$ Scale $$₹12$$ Crayons $$₹26$$

Here the question is 'What are all the maximum possible items you can buy with the amount you have?'

It will take a little more time if you do the exact calculation and finding the answer.

But you can find the answer to this question within a few seconds if you go by estimation.

The rounding off values of the provided items are $$₹50, ₹20, ₹10, ₹30$$. This adds up to $$₹110$$.

As we have $$₹100$$, leave the item corresponds to the estimated number $$₹10$$. That is, leave the scale which costs $$₹12$$.

Through this, it is clear that not just a single number is always estimated, but also we can estimate the outcomes of numbers.
Important!
There is no separate procedure to estimate the outcomes of numbers, and it depends on the required accuracy and how quickly we need the estimated answer. Also, the estimated answer should not affect the problem.