An estimation is just a rough guess; we turn the exact numbers into clean approximations to make them easier to work with, and that lets us get a good ball-park for the actual answer.
For example:
1) 1.64 / 22
1.64 is awfully close to 1.60, and 22 is nearest to 20, so we can use those rounded values to find that
1.64 / 22 ~= 1.60 / 20 = 0.08
Similarly, we have
2) 123.8 / 62 ~= 120.0 / 60 = 2
3) 6.15 / 31 ~= 6.00 / 30 = 0.2
