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Enigmas & puzzles

By Ian Stewart   February 2006

Howduzzi’s tricky triangle

The stage magician Henry Howduzzi was demonstrating his new trick to his assistant Doris. “It uses a special pack of cards,” he explained, “numbered consecutively from 1 to 15. I lay them out like this—except that in the real trick they are face down.”

“This arrangement is what makes the trick work. Each card is the difference between the two cards immediately below it, to its right and left. Except for the cards in the bottom row, of course.”

His assistant looked puzzled.

“For instance, Doris, the top card is 1, and those below…

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