Two tains of equal lengths 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 travelling in opposite direction?
Select:	A) 12	B) 14	C) 16	D) 20	E) 18
Sol.   Speed of the first train = [120 / 10] m/sec = 12 m/sec.   Speed of the second train = [120 / 15] m/sec = 8 m/sec.   Relative speed = (12 + 8) = m/sec = 20 m/sec.   ∴ Required time = (120 + 120) / 20= 
<gadget id="calculator">(120 + 120) / 20</gadget>
<output>12</output>
12 sec.   12nswer 12
<result>A</result>
Two tains of equal lengths 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 travelling in opposite direction?