The Chinese Remainder Theorem

In the 3rd century, Sun Tzu posed a puzzle: 'I have a number. Divide it by 3, the remainder is 2; by 5, the remainder is 3; by 7, the remainder is 2. What is the number?' The answer is 23 — and the general method behind it became the Chinese Remainder Theorem, one of the oldest results in number theory and now a quiet workhorse of modern computing.