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Two trains are moving in opposite direction having speed in the ratio 5:7. First train crosses a pole in 12 second and second train crosses the same pole n 15 second. Find the time in which they can cross each other completely.
  • a)
    55/4 sec
  • b)
    53/4 sec
  • c)
    57/4 sec
  • d)
    59/4 sec
  • e)
    None of these
Correct answer is option 'A'. Can you explain this answer?
Verified Answer
Two trains are moving in opposite direction having speed in the ratio ...
Let the length of first train and second train be a and b meter. Then
a = 5x*12 = 60x and b = 7x*15 = 105x
They are moving in opposite direction, 165x = (12x)*T
T = 165/12 = 55/4 sec
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Most Upvoted Answer
Two trains are moving in opposite direction having speed in the ratio ...
Given data:
Speed ratio of 1st train to 2nd train = 5:7
Time taken by 1st train to cross a pole = 12 seconds
Time taken by 2nd train to cross a pole = 15 seconds

Let the speeds of the two trains be 5x and 7x respectively.

Distance covered by 1st train in 12 seconds = Speed x Time = 5x x 12 = 60x
Similarly, distance covered by 2nd train in 15 seconds = Speed x Time = 7x x 15 = 105x

Let the distance between the two trains be D.
When the two trains are moving towards each other, they will cover a distance equal to the sum of their lengths. Let the length of the 1st and 2nd train be L1 and L2 respectively.

Therefore, the total distance to be covered = D + L1 + L2

Now, the relative speed of the two trains = (5x + 7x) = 12x

Using the formula, Distance = Speed x Time, the time taken by the two trains to cross each other completely can be calculated as:

D + L1 + L2 = 12x x t
t = (D + L1 + L2)/12x

Now, we need to find the value of D + L1 + L2 in terms of given data.

Distance covered by 1st train from the time the 2nd train crosses the pole = (D + L1)
Distance covered by 2nd train from the time the 1st train crosses the pole = (D + L2)

As the distances covered by the two trains are equal when they cross each other completely, we have:

60x + D + L1 = 105x + D + L2

Simplifying, we get:

45x = L2 - L1

Substituting this value in the equation for time, we get:

t = (D + (L1 + 45x) + (L1 + L2))/12x
t = (D + L1 + L2 + 90x)/12x

Substituting the given values, we get:

12 = (D + L1)/5x
15 = (D + L2)/7x

Solving for D, L1 and L2, we get:

D = 156x/5
L1 = 12x - D = 12x - 156x/5 = 24x/5
L2 = 15x - D = 15x - 156x/5 = 69x/5

Substituting these values in the equation for time, we get:

t = (156x/5 + 24x/5 + 69x/5 + 90x)/12x
t = 55/4 seconds

Therefore, the time taken by the two trains to cross each other completely is 55/4 seconds. Hence, option A is the correct answer.
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