India, July 14 -- By a happy coincidence, just weeks after we discussed digital roots and how they can help in determining if a number is a perfect square, I came across the concept again. To recap, the digital root is the result of repeatedly adding the digits of a number until you are left with a single digit. For example, the digital root of 2697 is obtained by adding 2 + 6 + 9 + 7 = 24, then 2 + 4 = 6.

The late American puzzler and author Martin Gardner describes a card trick based on digital roots. The trick was invented by Stewart James, a magician from Ontario. It is very easy to see why it works, but deducing that should be fun. You may also enjoy playing the trick on an audience.

#Puzzle 151.1

From a standard deck of 52, a mag...