Death From a Top Hat, by Clayton Rawson, contained a super puzzle as an example of perfect misdirection. With permission from the author of that excellent murder mystery of magicians, by a magician, and for magicians, it is given here for those who pride themselves on common sense, especially mathematicians.

X is the center of the circle. The distance AB is 5 inches and the distance BC is 9 ½ inches. What is the diameter of the circle? The time limit is one minute flat for a Jinx IQ of “quite a bit above normal”. You’ll do better if you forget your geometry and your calculus.

For those who peek when they can, the answer is to be found down below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SOLUTION TO PUZZLE FROM A TOP HAT

The diameter of the circle is 19 inches. Draw in the other diagonal of the rectangle. See it now ? The undrawn diagonal of any rectangle are always equal (you knew that), the radius is therefore 9 ½ inches and the diameter twice that. The five inch distance is pure misdirection.