Suppose train P crosses train Q in 55 sec when travelling in same direction then, Determine the length of train P. Statement I : Speed of train Q is 36 m/sec and it crosses a tree in 10 sec. Statement II: Train P crosses a tree in 35 sec.

ATQ, Statement I: Length of train Q = 36 × 10 = 360 metres We cannot Find the speed of train P. So, data in statement I alone is not suicient to answer the question. Statement II: We cannot ind the speed of trains P and Q. So, data in statement II alone is not sufficient to answer the question. Combining Statements I and II: Length of train Q = 360 metre Let the speed of train P be 'a' m/sec Length of train P = 35a meters Train P crosses train Q in 55 sec when travelling in same direction.  So,a – 36 = (360 + 35a)/55 Or, 55a – 1980 = 360 + 35a Or, a = 117 Length of train P = 35 × 117 = 4095 metres So, the data in both statements I and II together are necessary to answer the question.