Which one is good for this ques ..lcm or hcf ??

Question

2 model trains travel round a double track.1 train completes the curcuit in 12s; the other in 15s. if they start side by side, how long will it be before the are side by side again??

Answer 1

Three seconds because the difference between Train A and Train B is 3 seconds in arrival time. I'am very glad if you donate me 10 points because I really want to reach the Second Level.

Answer 2

The answer: my guess would be 60s. That's the LCM, isn't it?

The hcf (I'm assuming that means Highest Common Factor, because in my country, we call it GCF which means Greatest Common Factor) is 3, isn't it?

Well, the 15s train can never catch up to the 12s train so the only possible time they are side by side is when the 12s train and the 15s train meet at their starting point. It will take the 12s train 5 revolutions around the circuit and it will take the 15s train 4 revolutions until they are side by side again at their starting point.