India, July 21 -- We learn some properties of numbers early in life, such as all products of 9 having a digital root of 9, and all single-digit multiplications with 37 leading to products with three repeating digits. Other properties may pass unnoticed for years until they are presented to us in the face, and we wonder why we hadn't noticed it all along.

The following puzzle exploits a property I had not noticed until I first read and solved the puzzle some three decades ago. Martin Gardner was particularly fond of it, and credits it to two readers who brought it to his notice separately. My version tweaks the original described by Garden. The puzzle may seem to lack enough information when you first read it, but think a bit and you will...