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Bunuel
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Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
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If two trains with same speed take different time to cross a post, it essentially means the lenghts of train were different

Length of train A=D1
Length of train B=D2

Speed of train A=B=\(\frac{D1}{10}\)=\(\frac{D2}{15}\)

\(\frac{D1}{10}\)=\(\frac{D2}{15}\)
Or we can say D1=\(\frac{10D2}{15}\)

Time take for trains to cross= Total distance (D1+D2)/Relative speed
Converting everything in terms of D2, we have (\(\frac{25D2}{15}\))/(\(\frac{2D2}{15}\))=12.5 Seconds
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Speed of the trains be V
Length of train1 be 10v
Length of train2 be 15v

time taken by train for travelling in opp direct to cross each other= length of train / relative speed of train

(10v+15v)/2v
25/2=12.5 (option C)
Bunuel
Two trains traveling at the same speed take 10 seconds and 15 seconds respectively to cross a telegraph post. What is the time taken by the trains to cross each other if traveling in opposite directions?

A. 6 seconds
B. 12 seconds
C. 12.5 seconds
D. 25 seconds
E. 30 seconds


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You don't even need to solve, the answer has to be between 10 and 15 seconds, only choice C works.
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