Correct Answer - Option 4 : 140 km/hr
Given:
Length of train 1 = 157 m
Length of train 2 = 123 m
Time taken to cross each other in opposite direction = 7.2 s
Concept used:
Time taken by trains to cross each other in the opposite direction, t = (L1 + L2)/(S1 + S2)
Where S1 and S2 are speeds of trains 1 and 2 respectively.
s m/s = s × (18/5) km/hr
Calculation:
7.2 = (157 + 123)/(S1 + S2)
⇒ (S1 + S2) = 280/7.2 m/s
⇒ 280/7.2 × (18/5) = 140 km/hr
∴ The combined speed of both trains is 140 km/hr.