It is given that A train 150 meters long crosses a milestone in 15 seconds Length of train(L) = 150 m Time taken to cross milestone(T) = 15 sec Let speed of this train be = S m/sec So using , Distance = Speed x Time 150 = S x 15 S = 10 m/sec Now it is mentioned that this train crosses another train of the same length travelling in the opposite direction in 12 seconds let the speed of another train be Y m/sec As both trains are moving in opposite direction so relative speed = (Y + 10) m/sec So , (150+150) = (Y + 10) x 12 15 = Y + 10 Y = 5 m/sec OR (5 x $$frac{18}{5}$$ ) = 18 km/hr