Two trains cross a pole in 4 sec and 6 sec respectively. Running in same direction in how much time will they take to cross each other if the ratio of their speed is 7 : 9.

41 Sec

Ratio of          train 1    :    train 2 

Speed                7       :       9   

Distance = Speed × Time 

train 1 takes 4 sec to cross a pole , 

Length of train 1 = 4 × 7R = 28R

train 2  takes 6 sec to cross a pole , 

Similarly, legth of train 2 = 6× 9R = 54R

Total distance = 82R  meter 

As trains moves in same direction . so relative speed = (9 - 7 )R   =  2R m/sec

Time taken by trains to cross each other = \(\frac{82R}{2R}\) =  41 sec.