UPSC Prelims 2014·CSAT·Quantitative Aptitude·Number System

A bell rings every 18 minutes. A second bell rings every 24 minutes. A third bell rings every 32 minutes. If all the three bells ring at the same time at 8 o'clock in the morning, at what other time will they all ring together?

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Last updated 8 Jul 2026, 4:39 pm IST
  1. A12:40 hrs
  2. B12:48 hrsCorrect
  3. C12:56 hrs
  4. D13:04 hrs

Explanation

To find when all three bells will ring together again, we need to find the Least Common Multiple (LCM) of their individual ringing intervals.

  1. Identify the intervals: The bells ring every 18 minutes, 24 minutes, and 32 minutes.

  2. Calculate the LCM of 18, 24, and 32:

    • Prime factorization:
      • 18 = 2 x 3^2
      • 24 = 2^3 x 3
      • 32 = 2^5
    • LCM = 2^5 x 3^2 = 32 x 9 = 288 minutes.
  3. Convert the LCM from minutes to hours and minutes:

    • 288 minutes / 60 minutes/hour = 4 hours and 48 minutes (since 4 x 60 = 240, and 288 - 240 = 48).
  4. Add this duration to the initial ringing time:

    • Initial time: 8:00 AM
    • Add 4 hours 48 minutes: 8:00 + 4:48 = 12:48 PM.

Therefore, all three bells will ring together again at 12:48 hrs.

Analyzing the options: A) 12:40 hrs - Incorrect. B) 12:48 hrs - Correct, as it matches our calculation based on the LCM. C) 12:56 hrs - Incorrect. D) 13:04 hrs - Incorrect.

The final answer is B.

Quantitative Aptitude: A bell rings every 18 minutes. A second bell rings every 24 minutes. A third bell rings every 32 minutes. If all the thr

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