how do you estimate

example

$$215*98\approx 200*100\approx 20,000$$

Generally, you would estimate by rounding all the numbers that affect the solution.

This would usually be used to make a quick but not accurate answer. Sometimes I use an estimate to double check that my accurate answer is in the correct ball park. If my estimate and my accurate answer are very close then I can be a bit more confident that I am correct, if they are considerably different then I know to recheck both answers to see where I went wrong.