Q. 64.2( 80 Votes )

The traffic lights at three different road crossings change after every 48 seconds, 72 seconds and 108 seconds respectively. If they change simultaneously at 7 a.m., at what time will they change simultaneously again?

Answer :

Time-period after which these lights will change = LCM of 48, 72 and 108


LCM is the lowest common multiple. LCM of two numbers It is the smallest number that is divisible by both of them.



LCM = 2 × 2 × 2 × 2 × 3 × 3 × 3 = 432


The lights will change together after every 432 seconds.


60 seconds = 1 minute


432 seconds = 432/60 min



= 7 minutes and 12 seconds

Total time will be 7 am + 7 min + 12 sec.


Hence these lights will change simultaneously again at 7:07:12 am.

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