Description:  Here’s a fun onlineonly book that will help you do algebra in your head. Useful for any math or science student.
From the site:
 Many mathematicians and engineers can look at an equation, think a moment, and know its solution. Some can do very complex math this way. Most people can learn to do this. By using certain mental abilities you have in ways described in this book, you can do things mathematically that may have not seemed possible before.
 In this book, we focus on algebra; you learn to solve algebra equations mentally. It is necessary that you have already learned algebra in some way (probably through a math course). What you learn here is to do certain things that allow you to do algebra internally. If you are just learning algebra (for instance, if you are now taking a course in school), you will begin to find this book useful as soon as you have some experience solving equations. (You will mainly be working with what are called singular equations, which are explained below. For now you’ll ignore the other kind, called plural equations.)
