1. 

What is the length of a running train crossing another 180 metre long train running in the opposite direction?

I. The relative speed of the two trains was 150 kmph.

II. The trains took 9 seconds to cross each other.

A. I alone sufficient while II alone not sufficient to answer
B. II alone sufficient while I alone not sufficient to answer
C. Either I or II alone sufficient to answer
D. Both I and II are not sufficient to answer
E. Both I and II are necessary to answer

Answer: Option E

Explanation:

Let the two trains of length a metres and b metres be moving in opposite directions at u m/s and v m/s.

Time taken to cross each other = (a + b) sec.
(u + v)

Now,

b = 180, u + v = 150 x 5 m/sec = 125 m/sec.
18 3

9 = a + 180
(125/3)

a = (375 - 180) = 195 m.