The traffic lights at three different road crossings change every $24$ seconds, $36$ seconds, and $72$ seconds respectively. If they all change simultaneously at $8 : 20 : 00$ a.m., then they will again change simultaneously at

8 : 21 : 12

a.m.

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8 : 20 : 12

a.m.

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8 : 22 : 00

a.m.

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8 : 21 : 12

p.m.

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Solution

The correct option is A $8 : 21 : 12$ a.m.
Interval of change $= (L.C.M. of 24, 36, 72) = 72$ seconds.
So, the traffic lights will change after every $72$ seconds, i.e. $1$ minute $12$ seconds.
So, the next simultaneous change will take place at $8 : 21 : 12$ a.m.

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The traffic lights at three different road crossings change every 24 seconds, 36 seconds, and 72 seconds respectively. If they all change simultaneously at 8:20:00 a.m., then they will again change simultaneously at

The correct option is A 8:21:12 a.m.Interval of change =(L.C.M. of 24,36,72)=72 seconds. So, the traffic lights will change after every 72 seconds, i.e. 1 minute 12 seconds. So, the next simultaneous change will take place at 8:21:12 a.m.

The traffic lights at three different road crossings change every $24$ seconds, $36$ seconds, and $72$ seconds respectively. If they all change simultaneously at $8 : 20 : 00$ a.m., then they will again change simultaneously at

The correct option is A $8 : 21 : 12$ a.m.
Interval of change $= (L.C.M. of 24, 36, 72) = 72$ seconds.
So, the traffic lights will change after every $72$ seconds, i.e. $1$ minute $12$ seconds.
So, the next simultaneous change will take place at $8 : 21 : 12$ a.m.