Subject: General Aptitude · Chapter: Problems on Trains · Exam: · Marks: · Difficulty:
What is the length of the train X? I. Train X crosses a telegraph post in 20 seconds II. Train X crosses a platform of length 800 m in 100 seconds. III. Train X passes through a tunnel 400 m long in 60 seconds.
(a)I and either II or III only
(b)II and III only
(c)II and either I or III only
(d)III and either I or II only
(e)Any two of the three
Answer
Answer (as printed): E
Explanation
Time taken to cross a pole $$\begin{aligned} & =\frac{\text { Length of train }}{\text { Its speed }} \Rightarrow 20=\frac{l}{\text { speed }} \\ \Rightarrow \text { speed } & =\frac{l}{20} \end{aligned}$$ Time taken to cross a platform $=\frac{(l+800)}{\text { speed }}$ $$\Rightarrow 100=\frac{(l+800)}{\text { speed }} \Rightarrow \text { speed }=\frac{(l+800)}{100}$$Time taken to pass through a tunnel $=\frac{(l+400)}{60}$ $$\Rightarrow 60=\frac{(l+400)}{\text { speed }} \Rightarrow \text { speed }=\frac{(l+400)}{60}$$ Equating any two out of three will give us $l$. ∴ Correct answer is (e).
Explanation as extracted from the printed page; notation may be imperfect.