How Math Explains the World

1 annotation • Mar 2024 • data

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.