Subject: General Aptitude · Chapter: Problems on Trains · Exam: · Marks: · Difficulty:
What is the speed of a running train which takes 9 seconds to cross a signal post? I. The length of the train is 90 metres. II. The train takes 27 seconds to cross a platform of 180 metres.
Answer
SELF-PRACTICE — the source book printed no answer.
Nothing is invented here, so this question has no answer on record.
Explanation
Speed of the train $=\frac{\text { Length of the train }}{\text { Time taken to cross the post }}$ $$=\frac{90}{9} \mathrm{m} / \mathrm{s}=10 \mathrm{m} / \mathrm{s} .$$ Thus, I alone gives the answer. Time taken to cross a platform $$\begin{aligned} & =\frac{(\text { Length of train }+ \text { Length of platform })}{\text { Speed of the train }} \\ \Rightarrow \text { Speed } & =\frac{(l+180)}{27} . \end{aligned}$$ But, $l$ is not given. So, speed cannot be obtained. So, II alone does not give the answer. ∴ The correct answer is (a).