In mathematics, estimation means approximating a quantity to the required accuracy. This is obtained by rounding off the numbers involved in the calculation and getting a quick and rough answer.
To approximate numbers,
We can give the following examples
Example 1

We can approximate the numbers as follow

Similarly,

Thus, we will have

Therefore, the approximated value is 18
The actual answer is

We can look at another example

Thus

The actual answer is

In both cases, we can see that the approximated values and the actual values are not so far from each other.