This is a great book if you have not found ways to perform calculations in your head.  These tricks and techniques will give you the tools you will need, but it requires practice (read: homework) to make them stick, and to make them easier.<br><br>However, if you ever find yourself without a calculator, or if you want to race someone on theirs, this will help you figure out these problems.  I have started doing most multiplication problems in my head instead of on a calculator.