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Chapter 1 Class 10 Real Numbers
Serial order wise

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Question 3 (Introduction) Three alarm clocks ring their alarms at regular intervals of 20 min, 25min and 30 min respectively. If they first beep together at 12 noon, at what time will they beep again for the first time ? (a) 4:00 pm (b) 4 :30 pm (c) 5:00 pm (d) 5 :30 pmLet the bells ring at 1, 2, 3 minutes At 6 minutes, all of them ring together. Now, this 6 is actually the LCM of 1, 2, 3 So, Time when they ring together = LCM of 1, 2, 3 Question 3 Three alarm clocks ring their alarms at regular intervals of 20 min, 25min and 30 min respectively. If they first beep together at 12 noon, at what time will they beep again for the first time ? (a) 4:00 pm (b) 4 :30 pm (c) 5:00 pm (d) 5 :30 pmHere, Time when they next ring together = LCM of 4, 7 and 14 Finding LCM ∴ LCM = 2 × 2 × 3 × 5 × 5 = 4 × 3 × 25 = 300 Thus, the three bells ring together after 300 minutes, i.e. after 5 hours Hence, Time when the three bells ring together is 5:00 pm So, the correct answer is (c)

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Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 12 years. He provides courses for Maths and Science at Teachoo.