Given: The ratio of speed of trains = 5 : 7 Both trains take 4 seconds to cross the telegraph post Formula Used: Distance = Speed × Time Calculation: Let the speed of Train 1=S1=5am∕sec. Let the length of Train 1=L1 Let the speed of Train 2=S2=7am∕sec. Let the length of Train 2=L2 Time taken for the train to cross the telegraph post = (Length of the train)/(Speed of the train) For Train 1, 4=
L1
5a
⇒ L1=20a For Train 2, 4=
L2
7a
⇒ L2=28a Time taken by both the trains to cross each other completely =(Sum of length of the trains)/(sum of speed of the trains) {∵ Trains are crossing each other, they travel in opposite directions} ⇒ Time taken =