How Math Explains the World
James D. Stein
Preface
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Any two-digit 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)=9T-9 U=9(T-U), which is clearly divisible by 9.
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