Two trains are moving in opposite directions on two parallel tracks. How many seconds will they take to cross each other if the sum of their lengths is 357m, and the sum of their speeds is 51m/s.?
The total distance is equal to the sum of the lengths of the trains, so s = 357. This distance has to be covered at a net relative speed equal to the sums of the speeds of the two trains, so v = 51. The time will be distance/speed = $357/51$ = 7 sec.