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Solutions to problems below

Mind Stimulating Exercises


Raymond Smullyan (1978). What is the Name of this Book: Riddle of Dracula and Other Logical Puzzles. Prentice-Hall: Englewood Cliffs, 1978.

  1. A man was looking at a portrait. Someone asked him, “Whose picture are you looking at?” He replied: “Brothers and sisters have I none, but this man’s father is my father’s son.” (“This man’s father” means, of course, the father of the man in the picture.) Whose picture was the man looking at?

  2. What happens if an irresistible cannonball hits an immovable post? By an irresistible cannonball we shall mean a cannonball which knocks over everything in its way. By an immovable post we shall mean a post which cannot be knocked over by anything. So what happens if an irresistible cannonball hits an immovable post?

  3. Two men were being tried for murder. The jury found one of them guilty and the other one not guilty. The judge turned to the guilty one and said: “This is the strangest case I have ever come across! Though your guilt has been established beyond any reasonable doubts, the law compels me to set you free.” How do you explain this?

  4. Two American Indians were sitting on a log- a big Indian and a little Indian. The little Indian was the son of the big Indian, but the big Indian was not the father of the little Indian. How would you explain this?

  5. A man is 100 yards due south of a bear. He walks 100 yards due east, then faces due north, fires his gun due north, and hits the bear. What color was the bear?

  6. Two American coins add up to thirty cents, yet one of them is not a nickel. What coins are they?

  7. Those of you who know anything about Catholicism, do you happen to know if the Catholic Church allows a man to marry his widow’s sister?

  8. Those of you who are interested in questions of good grammatical usage, is it more correct to say the yolk is white or the yolk are white?

  9. A train leaves from Boston to New York. An hour later, a train leaves from New York to Boston. The two trains are going at exactly the same speed. Which train will be nearer to Boston when they meet?

  10. On a certain house, the two halves of the roof are unequally pitched; one half slopes downward an angle of 60 degrees and the other half at an angle of 70 degrees. Suppose a rooster lays an egg right on the peak. On which side of the roof would the egg fall?

  11. A certain street contains 100 buildings. A sign-maker is called to number the houses from 1 to 100. He has to order numerals to do the job. Without using pencil and paper, can you figure out in your head how many 9’s he will need?

  12. A certain snail takes an hour and a half to crawl clockwise around a certain race track, yet when he crawls counter-clockwise around that same racetrack it takes him only ninety minutes. Why this discrepancy?

  13. If an airplane crashes right on the border of the United States and Canada, in which country would you bury the survivors?

  14. A certain Mr. Smith and his son Arthur were driving in a car. The car crashed; the father was killed outright and the son Arthur was critically injured and rushed to a hospital. The old surgeon took a look at him and said, “I can’t operate on him; he is my son Arthur!” How do you explain this?