Question

Two persons are walking beside a railway track at respective speeds of 2 and 4 km per hour in the same direction. A train came from behind them and crossed them in 90 and 100 seconds, respectively. The time, in seconds, taken by the train to cross an electric post is nearest to

• A. 87
• B. 82
• C. 75
• D. 78

Answer 1

Answer: (B)

82

Answer 2

The correct option is (B): $82$

Let the length of the train be l and its speed be $s$.

Given $\frac{l}{(s-2)×\frac{5}{18}} = 90; \frac{l}{(s-4) ×\frac{5}{18}} = 100$

$⇒ 90(s - 2)×\frac{5}{18} = 100(s - 4)×\frac{5}{18} ⇒ s = 22$

∴ Length of the train =$500$ m.

Hence the required time to cross a lamp post = $\frac{500}{22×\frac{5}{18}}$

i.e., $81.81$ (or) $82$ sec.