The traffic lights at three different road crossings change after every 48 seconds, 72 seconds and 108 seconds respectively.
Question:

The traffic lights at three different road crossings change after every 48 seconds, 72 seconds and 108 seconds respectively. If they all change simultaneously at 8 a.m. then at what time will they again change simultaneously?

Solution:

We find the LCM of 48, 72 and 108 first to get the time after which they will blink together again. 

Hence, LCM $=2 \times 2 \times 2 \times 2 \times 3 \times 3 \times 3=432$

So, they will blink again at 432 seconds past 8:00 am

or, $\frac{432}{60}=7$ minutes and 12 seconds past $8: 00$ am

So, the time will be 08:07:12 hrs

 

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