The grid of numbers below is WormWorld, and it's a tough place to live. The worms that inhabit it are all born with nasty allergies. The first time they come in contact with a number, their immune systems are overstimulated; if they are exposed to that number a second time, they die of anaphylactic shock.
A worm can start crawling on any square in WormWorld, and it can then move horizontally or vertically but not diagonally. What is the longest path a worm can take without dying?
Wait. Don't cheat. Don't click on the solution until you've given this a try or had a friend try to solve the puzzle.
"Click Here for Solution"
My WormWorld puzzle was first published in the November 1994 issue of Discover Magazine, and is reprinted here with permission of the publisher. My books "Keys to Infinity" and "Wonders of Numbers" goes into further mathematical detial and describes the Internet WormWorld Tournament I held, inviting people to compete against a computer program.
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