There are 3 consecutive traffic lights which turn "green" after every 36, 42 and 72 seconds. They all were at "green" at 9:00 AM. At what time will they all turn "green" simultaneously?

L.C.M. of 36, 42, and 72 = 504

 So, after 504 seconds ( 8 minutes 24 seconds) , they all will turn green simultaneously. Therefore after 9 a.m. ,they all will turn green at 9 : 08 : 24 a.m.  

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