The metro service has a train going from A to B and from B to A every

I am assuming the question means that we are travelling by the 12'o clock train from A to B.
So we will reach B at 4:30 pm.

A couple of things to note:
- We start from A toward B at 12'o clock. So the 11'o clock train from A to B will always be 1 hr ahead and 1' o clock train will always be 1 hr behind (since all trains have the same speed). So we don't have to bother about meeting any trains going from A to B.

- We will meet every train that is anywhere on the track from B to A at any time in between 12 to 4:30.

The train that starts at 7:00 am from B will reach A at 11:30 am and we will not meet it. We will meet the train that starts at 8:00 am and every subsequent train till the one that starts at 4:00 pm from B to A.
So we will meet 9 trains going from B to A.

Answer (B)

It's best if you can visualise such Q statements first, before getting hold of your pen and paper.

And carefully go through EVERY information that's been provided:

The first trains start at 6 AM; it takes 4:30 hours for the trains to complete their journies

Now, simply working with this information we see that the train that left its source at 6:00 AM will reach its destination at 10:30, and the one leaving its source at 7:00 AM will reach its destination at 11:30 AM (both complete their journies even before we start ours - so basically they're irrelevant to us)

NOW, the next train leaves at 8:00 AM and will only reach its destination by 12:30 PM. Aha! We'll be in transit at this time and will pass this train during our journey - so that's train #1

Let us now look at all trains travelling towards the city that we departed from (leaving at or after 8:00 AM)

BUT, before that, let us see what time do we get off from the train, post which none of the trains is relevant to us. For that, we start off at 12:00 - take 4:30 hours to complete our journey - and get off at 4:30. So basically, any train leaving after 4:30 does not concern us since we would've already completed our journey before it starts its journey and hence, we will not "pass" the train.

So, we will be passing all trains departing from their source station between 8:00 AM to 4:30 PM, i.e. the ones departing at 8:00 AM, 9:00 AM, 10:00 AM, 11:00 AM, 12:00 PM, 1:00 AM, 2:00 PM, 3:00 PM, 4:00 PM

9 trains, in total, and hence, (B) is our answer.

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