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The wheels revolve round a common horizontal axis. They make 15, 20 and 48 revolutions in a minute respectively. Starting with a certain point on the circumference down wards. After what interval of time will they come together in the same position?

Correct answer: A. 1 min

  • A. 1 min
  • B. 2 min
  • C. 3 min
  • D. None

Explanation

After t minutes, the wheels make 15t, 20t and 48t revolutions. They are all back at the original position when these three quantities are whole numbers; the first such time is t = 1 minute, giving 15, 20 and 48 complete revolutions. The likely mistake is option b, which waits for two minutes even though all three wheels already coincide after one minute.

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