I believe everyone can learn mental math. In this short book, I am going to share 10 simple rules on how mental math works in practice. Unlike many other books, I will not burden you with tons of exercises. Rather I will give you several tricks you can use right there. For some tricks, I will teach you method that you can calculate quickly. In computer science, those are called the "algorithm". e.g. Trick 1 "Squares of number ended with 5" gives you an impressive party trick such that you can calculate the squares of two or even three digit number instantly. In some cases, I tell you the principles of mental math. Why sometimes certain rules take a long time to execute. e.g. Trick 5 "Always working out the largest digit first in addition". This allows you to call of or understand seemingly "smart" mental mather. The truth, not many of them are that smart once you understand their trick. In any case, sit back and I hope you enjoy.