Question

A train running at a certain speed crosses another train running in the opposite direction in $4.8$ seconds. to find out the speed of the first train , which of the following information P and Q is sufficient ?

$P:$ Then length of the first train

$Q:$ The length of the second train

• A. only $P$ is sufficient
• B. only $Q$ is sufficient
• C. either $P$ or $Q$ is sufficient
• D. Both $P$ and $Q$ are needed
• E. Both $P$ and $Q$ are not sufficient
  

Answer 1

Answer: (E)

Both $P$ and $Q$ are not sufficient

Let two trains of lengths $a$ and $b$ metres be moving in opposite directions at $u$ m /s and $v$ m/s

Time taken by he trains to cross each other $=\frac{a+b}{u+v}$ sec

$\therefore \frac{a+b}{u+v}=4.8$

In order to find $u$, we must know $a,b$ and $v$,

i.e. , length of first train , length of second rain and the speed of the second train .

Thus , $P$ and $Q$ are not sufficient .

$\therefore$The correct answer is option$\left(e\right)$