How Math Explains the World
James D. Stein
Preface

Any twodigit number can be written as 10T+U, where T is the tens digit and U the units digit. Reversing the digits gives the number 10U+T, and subtracting the second from the first yields 10T+U(10U+T)=9T9 U=9(TU), which is clearly divisible by 9.