Estimation is a mental process of coming up with an answer that is relatively close, to allow decisions to be made. The types of estimation are quantity, computation and measurement.
An estimation strategy is simply coming up with a good estimate based on evidence and already known facts. Coming to a conclusion as close to the real answer as you can. It can also be called an educated guess.
It says 2 significant figures so the least it can possibly be is 0.5 less and the most it can be is 0.5 more as if it was, lets say 20.4 it would round down to 20 not 21.
