Indian Institute of Foreign Trade 2012 Solved Paper

Speed of train 1 = 60 km/hr = 50/3 m/s
Speed of train 2 = 48 km/hr = 40/3 m/s
Thus, their relative speed = 90/3 = 30 m/s
Let, Length of train 1 = l
Thus, the length of train 2 will be 0.5 l
Time taken by them to cross each other = 15 s
Thus, $15\times 30=1.5\mathit{l}$
Thus, $\mathit{l}=300$
Train 1 takes 51s to cross a bridge
Let the length of the bridge be b
Thus, $\frac{50\times 51}{3}=300+\mathit{b}$
Hence, $850=300+\mathit{b}$
Hence, $\mathit{b}=550\mathit{m}$
Thus, option A is the correct answer.