1. 

What is the speed of the train?

I. The train crosses 300 metres long platform in 21 seconds.

II. The train crosses another stationary train of equal length in 19x(1/2) seconds.

III. The train crosses a signal pole in 9x(3/4) seconds.
A. I and II only
B. I and either II or III only
C. II and either I or II only
D. III and either I or II only
E. None of these.

Answer: Option B

Explanation:

Let the speed of the train be x m/sec.

Time taken to cross a signal pole

= Length of the train
Speed of the train

Time taken to cross a platform

= (Length of the train + Length of the Platform)
Speed of the train

I gives, 21 = (l + 300)/x;

II gives, 39/2 = 2l/x;

III gives, 39/4 = l/x.

Thus, (I and II) or (I and III ) give x.

Correct answer is (B)