Flashing Lights LCM

CAT 2001 Slot 1 · Quantitative Ability · Easy · Number Systems

This is an easy Quantitative Ability question from the CAT 2001 Slot 1 paper. It tests Number Systems. The full answer key and a step-by-step explanation are below — try it yourself first, then reveal the solution.

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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