QUESTION

CSAT

Easy

Maths

Prelims 2023

There are three traffic signals. Each signal changes colour from green to red and then from red to green. The first signal takes $25$ seconds, the second signal takes $39$ seconds and the third signal takes $60$ seconds to change the colour from green to red. The durations for green and red colours are same. At 2:00 p.m, they together turn green. At what time will they change to green next, simultaneously?

Select an option to attempt

Explanation

The three traffic signals turn from green to red at an interval of 25 seconds, 39 seconds and 60 seconds.

So, the time after which all the three traffic signals will turn red from green together = LCM of 25 seconds, 39 seconds and 60 seconds = 3900 seconds = 65 minutes = 1 hour 5 minutes

Now, durations for green and red colours are same. So, the time after which all the three traffic signals will turn green from red together = 1 hour 5 minutes So, the total time taken by all the three traffic signals to turn green together = 1 hour 5 minutes + 1 hour 5 minutes = 2 hours 10 minutes

Thus, the time after which all the three traffic signals will simultaneously change to green again = 2:00 p.m. + 2 hours 10 minutes = 4:10 p.m.

Hence, option (B) is correct.

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