Rounding and Estimation
This free, online textbook covers some basic functions of mathematics. "The main aim of this unit is to help you to do this in a sensible and fruitful way. Using a calculation to solve a problem involves four main stages: Stage 1: working out what calculation you want to do; Stage 2: working out roughly what size of answer to expect from your calculation; Stage 3: carrying out the calculation; Stage 4: interpreting the answer – Does it agree with the rough estimate? Does it make sense in terms of the original problem? This unit focuses on Stages 2 and 4."