Flashing Lights LCM

CAT 2001 Slot 1 · QA · Easy · Number Systems

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A red light flashes three times per minute and a green light flashes five times in 2 min2\text{ min} at regular intervals. If both lights start flashing at the same time, how many times do they flash together in each hour?

  1. A.

    30

  2. B.

    24

  3. C.

    20

  4. D.

    60

Answer

A

Explanation

Red light flashes every 60/3=20 s60 / 3 = 20\text{ s}. Green light flashes every 120/5=24 s120 / 5 = 24\text{ s}. They flash together at intervals equal to LCM(20,24)=120 s=2 min\text{LCM}(20, 24) = 120\text{ s} = 2\text{ min}. In one hour (60 min60\text{ min}), they flash together 60/2=3060 / 2 = 30 times.

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