Re: Two buses are travelling on the highway from X to Y .

Time for 1st bus travels from X to Y = 180/60 = 3h
Time for 2nd bus travels from X to Y = 180/30 = 4.5h

Because 1st bus traveled faster ==> 1st bus must meet 2nd when 1st bus went back from Y to X

t = time 2nd bus traveled to meet 1st bus
==> 2nd bus's distant = 40t
==> 1st bus's distant = 180 + 60(t - 3 + 1/4 - 1/4) because 1st bus started 15m (1/4h) earlier than 2nd bus, but waited 15m (1/4h) before traveling back to X.

We have equation: 40t + 60(t - 3) = 180
==> t = 3.6 h = 3h and 36 minutes

Question #1: How far from Y will the first Bus meet the second?
distant from Y = 60(3.6 -3) = 36km

Question #2: how much time will they meet each other after the departure of the first bus from X?
Time = time 2nd bus traveled to meed 1st bus + 15 minutes = 3h 36minutes + 15minutes = 3h 51 minutes.

Hence, A is correct.

Hope it helps.