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Railways PYQCBT 1 2016

RRB NTPC · Question 35

The traffic lights at four different road crossings change after every 15 sec, 18 sec, 27 sec and 30 sec respectively. If they all change simultaneously at 6 : 10 : 00 hours, then at what time will they again change simultaneously?

  1. 6 : 14 : 30 hours

  2. 6 : 40 : 00 hours

  3. 6 : 14 : 00 hours

  4. 10 : 40 : 00 hours

Show answer & explanation

Answer: (A) 6 : 14 : 30 hours

Explanation

The lights change together again after LCM(15, 18, 27, 30) seconds. 15 = 3 × 5, 18 = 2 × 3², 27 = 3³ and 30 = 2 × 3 × 5, so LCM = 2 × 3³ × 5 = 270 s = 4 min 30 s. Starting at 6:10:00, they next change together at 6:14:30.