Correct Answer - Option 1 : 60
Given:
Time taken by the train starting from A to reach B, after crossing = \(6\dfrac{1}{4}\) hours = 25/4 hours
Time taken by the train starting from B to reach A, after crossing = 9 hours
Speed of the train starting from A = 72 km/h
Concept used:
If two trains P and Q start at the same time from two points A and B towards each other and after crossing, they take "a" and "b" hours in reaching B and A respectively. Then
P's speed ∶ Q's speed = √b ∶ √a
Calculation:
Let the speed of the train starting from B = v km/h
\(\frac{v}{{72}} = \;\sqrt {\frac{{\left( {\frac{25}{4}} \right)}}{{\ 9}}} \)
⇒ \(\frac{v}{{72}} = \;\sqrt {\frac{25}{36}} \)
⇒ v/72 = 5/6
⇒ v = 60 km/h
∴ The speed of the train starting from B is 60 km/h.