A 150 m long train crosses a milestone in 15 seconds and a train of same length coming from the opposite direction in 12 seconds. The speed of the other train is?

Answer: Option D

Speed of first train = $$\frac{150}{15}$$ m/sec = 10 m/sec
Let the speed of second train be x m/sec
Relative speed = (10 + x) m/sec
∴ $$\frac{300}{10+\text{x}}$$  = 12
⇒ 300 = 120 + 12x
⇒ 12x = 180
⇒ x = $$\frac{180}{12}$$ = 15 m/sec
Hence, speed of other train
= $$\left(15\times \frac{18}{5}\right)$$  kmph
= 54 kmph