Train A crosses train B completely in 90 seconds, with the trains running in opposite directions. The lengths of trains A and B are 900 metres and 1800 metres respectively. If the speed of train A is 45 km/h, then in how much time can train B cross a stationary pole? (in seconds)

Length of train A = 900 metres

Length of train B = 1800 metres

Speed of train A = 45 km/h

= $$\frac{45000}{3600}$$ m/s

= 12.5 m/s

Train A crosses train B completely in 90 seconds, with the trains running in opposite directions.

relative time = $$\frac{Sum\ of\ the\ length\ of\ both\ trains}{Sum\ of\ the\ speed\ of\ both\ trains}$$

$$\frac{900+1800}{12.5+\ speed\ of\ train\ B}\ =\ 90$$

Time taken by train B to cross a stationary pole = $$\frac{Length\ of\ train\ B}{speed\ of\ train\ B}$$

= $$\frac{1800}{17.5}$$

= $$\frac{18000}{175}$$

= $$\frac{720}{7}$$ seconds