AFCAT Model Paper 4

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 =

(20a+28a)

(5a+7a)

⇒ Time taken =

48a

12a

∴ Time taken = 4 sec