Two tains of equal lengths take 10 seconds and 20 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 travelling in opposite direction?  Select:
A) 12
B) 14
C) 16
D) 20
E) 13.3
Sol.   Speed of the first train = [120 / 10] m/sec = 12 m/sec.   Speed of the second train = [120 / 20] m/sec = 6 m/sec.   Relative speed = (12 + 6) = m/sec = 18 m/sec.   ∴ Required time = (120 + 120) / 18 secc = 13.3 sec.   Answer 13.3
<result>E</result>
Two tains of equal lengths take 10 seconds and 20 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 travelling in opposite direction?