Two trains cross a pole in 4 sec and 5 sec respectively. Running in same direction in how much time will they take to cross each other if the ratio of their speed is 5 : 7.

27.5 sec

Ratio of          train 1    :    train 2 

Speed                5      :       7   

Distance = Speed × Time 

train 1 takes 4 sec to cross a pole , 

Length of train 1 = 4 × 5R = 20R

train 2  takes 5 sec to cross a pole , 

Similarly, legth of train 2 = 5× 7R = 35R

Total distance = 55R  meter 

As trains moves in same direction . so relative speed = (7 - 5 )R   =  2R m/sec

Time taken by trains to cross each other = \(\frac{55R}{2R}\) =  27.5 sec.