5Mental Math
Did you know that you could multiply many numbers by 10 just by adding a zero? And that you could divide many numbers by 100 by shifting a decimal point two places to the left? Those are just two of the shortcuts that can help you do calculations “in your head,” without needing paper and pencil or a calculator. In this chapter, we'll look at some of the tips and tricks for mental math. You may not need all of them, but you may find some helpful, and they're also good for amazing your friends. We'll be making reference back to Chapter 2, “Essential Arithmetic,” so review that if necessary.
1 FAST MULTIPLICATION
One of the laws of arithmetic is that any number multiplied by 1 is that number. The next time someone asks you to ...
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