Two trains are moving in same direction on two parallel tracks. How many seconds will they take to cross each other if the sum of their lengths is 306m, and the difference of their speeds is 51m/s?
The total distance is equal to the sum of the lengths of the trains, so s = 306. This distance has to be covered at a net relative speed equal to the difference of the speeds of the two trains, so v = 51. The time will be distance/speed = $306/51$ = 6s.