Question

Train A, running at the speed of 80 km/hr crossed train B, running at the speed of 70 km/hr in the opposite direction. Both trains finish crossing each other in 30 seconds. If the length of train A is 300 m, then the length of train B is:
1. 855 m
2. 950 m
3. 850 m
4. 750 m

Answer 1

Correct Answer - Option 2 : 950 m

Given:

Speed of train A = 80 km/h 

Speed of train B = 70 km/h 

Time taken in crossing = 30 seconds 

Length of train A = 300 m 

Formulas used:

Relative speed of trains (opposite direction) = S_A + S_B

Distance covered while crossing =  Length (Train A + Train B)

Time = Distance/Relative Speed 

Calculation:

Let the length of train B = b m 

30 seconds = (300 + b)/(70 + 80) km/h 

⇒ 30 seconds = (300 + b)/150 km/h 

⇒ 30 secs × 150 km/h × 1000m/3600 secs = 300 + b

⇒ 1250 = 300 + b 

⇒ b = 1250 - 300 = 950

∴ The length of train B = 950 m