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