OA Explanation:

Speed of Train A = 60 km/hr
Time to cross the post = 5 seconds = $\frac{5}{3600} = \frac{1}{720}$ hours
Length of Train A = $(60)(\frac{1}{720}) = \frac{1}{12}$ km

Speed of Train B = 90 km/hr
Time to cross the post = 6 seconds = $\frac{6}{3600} = \frac{1}{600}$ hours
Length of Train A = $(90)(\frac{1}{600}) = \frac{3}{20}$ km

Applying the concept of Relative Velocity:

Total Distance = $\frac{1}{2} + \frac{3}{20} = \frac{7}{30}$ km
Relative speed = $60 + 90 = 150$ km/hr
Time taken to cross each other = $\frac{7}{(30)(150)} = \frac{7}{4500}$ hr
i.e. $(\frac{7}{4500})(3600) = \frac{28}{5} = 5.6$ seconds