Circular Track Question

[thisso]

Hi All,

This Question is from GMAT club.

I got the ANS using the LCM of the 2 speeds. However, when I tried to solve it using relative speed, I don’t understand why I am supposed to subtract the 2 speeds. When they are going in opposite directions, don’t you add the speeds? Or is this rule different for circular tracks.

ANS is B BTW.

Thanks!!

[prakithgre]

Hmm the question doesn’t state when both trains meet or cross each other, they are asking a when both the trains reach a specific point. So the best way is to take the LCM and solve. Like I did by listing out the times each take to reach point S and picked a common time which is nothing but LCM