Subject: General Aptitude · Chapter: Problems on Trains · Exam: · Marks: · Difficulty:
What was the length of a running train crossing another 180 metre long train running in the opposite direction? I. The relative speed of the two trains was 150 kmph. II. The trains took 9 seconds to cross each other.
Answer
SELF-PRACTICE — the source book printed no answer.
Nothing is invented here, so this question has no answer on record.
Explanation
Let the two trains of length $a$ metres and $b$ metres be moving in opposite directions at $u \mathrm{m} / \mathrm{s}$ and $v \mathrm{m} / \mathrm{s}$. Time taken to cross each other $=\frac{(a+b)}{(u+v)}$ sec. Now, $b=180, u+v=\left(150 \times \frac{5}{18}\right) \mathrm{m} / \mathrm{sec}=\frac{125}{3} \mathrm{m} / \mathrm{sec}$ $$\Rightarrow 9=\frac{a+180}{(125 / 3)} \Rightarrow a=(375-180)=195 \mathrm{m} .$$ Thus, both I and II are necessary to get the answer. ∴ The correct answer is (e).