Question

There are three consecutive road crossings at which traffic lights change after every 48 seconds 72 seconds and 108 seconds respectively If the lights change simultaneously at 8 : 20 : 00 hours then at what time will they change again simultaneously?

Answer 1

The lights will change simultaneously after every interval = (L.C.M. of 48, 72 and 108) seconds
$\therefore$ LCM of 48, 72, 108
= $2\times 2\times 2\times 3\times 3\times 2\times 3=432$
432 seconds = 7 min and 12 seconds
$\therefore$ The next change will be at 8 : 27 : 12 hours