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  Lateral Thinking

Regular or Diet?
 
Q One teenager goes up to a food booth at a fair and says, "A cola, please." The man working the booth asks, "Regular or diet?" The teenager asks, "What's the difference?" and is told that the regular costs 90 cents but that the diet soda costs $1. The teenager says, "Give me a diet cola, please" and places a dollar on the counter.

Next, another teenager comes up to the booth and says, "A cola, please," placing a dollar on the counter. The man in the booth gives him a diet cola.

How did the man in the booth know which soda-regular or diet-the second teenager wanted?


 
A The second teenager put a dollar in change on the counter: 2 quarters and 5 dimes. If the teenager had wanted a regular cola, he would have put only 2 quarters and 4 dimes on the counter.

Source: Inspired by a puzzle in Mensa Presents Mighty Mind Boosters, P. Carter, J. Fulton, and K. Russell. New York: Barnes & Noble, 1996.