Wikijunior:The Book of Estimation/Numbers
In this unit, we will learn how to get the approximate value of numbers and in calculation. This covers the most basic concepts of approximation. The first chapter, Approximation of numbers, is the foundation of the rest of the book. If you don't understand just one bit, you will not understand the rest of the book at all. Luckily, the chapter is also quite easy.
In the second chapter, Accuracy and precision, we will learn the meaning of the words accurate and precise. We will learn about significant figures as well, and clear up some misconceptions you may have gotten while reading the first chapter. This chapter is also very important, especially when you start to learn about errors.
In the third chapter, Estimation in calculation, we will learn how to use the skills we learnt from the first two chapters to apply estimation in calculation. This part is relatively unrelated to the basics, but it is very helpful both in mathematics and in daily life.
Before you start 
Make sure you are familiar with the decimal system, and are able to count up to a billion. Read out these numbers aloud if you are not sure if you are able to:
Make sure you know how to do the four operations. Negatives will be rare in this section, and there will be no exponents or roots. Prepare a calculator as well as paper for calculation.
Also recall the meaning of words such as natural number, whole number, integer, fraction, decimal, negative and positive.