Question

Two trains of equal length take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time (in seconds) will they cross each other in opposite directions ? 

(a) 16 

(b) 15 

(c) 12 

(d) 10

Answer 1

(c) 12 seconds

Speed of the first train = \(\frac{120}{10}= 12 \) m/s

Speed of the second train = \(\frac{120}{15}= 8 \) m/s

∴ Required time = \(\frac{\text{Total distance}}{\text{Relative speed}}\)

= \(\frac{120+120}{12+8}\) = \(\frac{240}{20}\) seconds

= 12 seconds.