Question

It takes 15 seconds for a train travelling at 60 km/hr to cross entirely another train half its length and travelling in opposite direction at 48 km/hr. It also passes a bridge in 51 seconds. The length of the bridge is:

• A. 550 m
• B. 450 m
• C. 500 m
• D. 600 m

Answer 1

Answer: (A)

550 m

Let length of the train is X and length of the bridge is Y.
Speed of the First train = 60 km = (60 *5)/18 = 16.66 m/sec
Trains are moving in opposite direction, so their relative speed would be ,
S = 60 + 48 = 108 kmph = (108 *5)/18 = 30 m/sec
train crosses another train having half of its length in 15 sec . So, Train covered (X + X/2) = 3X/2 m in 15 sec.
Relative Speed = 3X/(2 *15)
3X/30 = 30 m/sec
X = 300 m
Length of the train = 300 m
Train crosses bridge in 51 sec. That is
Train needs to cover (300 + Y)m in 51 sec.
Speed of the train = (300 + Y)/51 m/sec
16.66 = (300 + Y)/51
850 = 300 + Y
Y = 850 - 300 = 550 m
Length of the bridge = 550 m.