A clock takes 30 seconds to strike six — how long to strike twelve?
A wall clock strikes six o'clock, and you time it: exactly 30 seconds pass between the first stroke and the last. At the same steady pace, how many seconds will the same clock take to strike twelve times at midnight?
Reveal the answer
66 seconds, not 60. Striking six times creates only five gaps between strokes (not six), so each gap is 30 divided by 5 = 6 seconds. Striking twelve times creates eleven gaps, so the total is 11 x 6 = 66 seconds. The trap is silently assuming the time scales with the number of strikes rather than the number of gaps between them. It's one of the puzzles in Boris Kordemsky's The Moscow Puzzles (1956), the best-selling Soviet puzzle collection later edited for English-language readers by Martin Gardner.
— Boris Kordemsky, The Moscow Puzzles — 1956 (English ed. 1972, ed. Martin Gardner)
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