The correct option of this question is (B).
Solution : If v (in km h-1) is the constant speed of the trains then the distance between the successive trains
= \(v × \frac {x}{60} = \frac {vx}{60}km\)
When a train moves in the same direction as that of the bus passenger,
\(=\frac {vx/60}{v-30} = \frac {20}{60}\)
Or, vx = 20(v-30) .......(i)
When a train moves in a direction opposite to the bus passenger,
\(\frac {vx/60}{(v+30)} = \frac {10}{60}\)
Or, vx = 10(v+30). .........(ii)
From eqns. (i) and (ii),
20(v-30) = 10(v+30)
Or, v =90 km h-1
From eqn. (i),
90x = 20(90-30) = 1200
Or, x = 13 min 20 s