VeritasPrepKarishma wrote:
fra wrote:
Two trains A and B starting from two points and travelling in opposite directions, reach their destinations 9 hours and 4 hours respectively after meeting each other. If the train A travels at 80kmph, find the rate at which the train B runs.
(A) 40
(B) 60
(C) 120
(D) 80
(E) 100
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Using Formula:
If two objects A and B start simultaneously from opposite points and, after meeting, reach their destinations in ‘a’ and ‘b’ hours respectively (i.e. A takes ‘a hrs’ to travel from the meeting point to his destination and B takes ‘b hrs’ to travel from the meeting point to his destination), then the ratio of their speeds is given by:
Sa/Sb = √(b/a)
i.e. Ratio of speeds is given by the square root of the inverse ratio of time taken.
Sa/Sb = √(4/9) = 2/3
This gives us that the ratio of the speed of A : speed of B as 2:3.
Since speed of A is 80 kmph, speed of B must be 80*(3/2) = 120 kmph
You can also do this question without using the formula. Check out this post for the method:
http://www.veritasprep.com/blog/2013/04 ... -formulas/Can you please explain this a little more?
First, nothing is given in the question as to what are the destinations of each train. Are we to assume that the starting point of A is destination of B and vice-versa?
Second, the question also doesn't say whether the trains started at the same time, can we just assume that they did?