Two trains of equal lengths take 10 sec and 15 sec respectively to cross a telegraph post. If the length of each train be 200 m, in what time will they cross other travelling in opposite direction? Choose one:
A) 17 sec  B) 12 sec  C) 16 sec  D) 15 sec  E) 20 sec
Speed of the first train = 120/10= 
<gadget id="calculator">120/10</gadget>
<output>12</output>
12 m/sec.   Speed of the second train= 120/5 = 8 m/sec.   Relative speed = 12 + 8= 
<gadget id="calculator">12 + 8</gadget>
<output>20</output>
20 m/sec.   Required time= (200 + 200)/20= 
<gadget id="calculator">(200 + 200)/20</gadget>
<output>20</output>
20 sec.   Answer:20 sec
<result>E</result>
Two trains of equal lengths take 10 sec and 15 sec respectively to cross a telegraph post. If the length of each train be 200 m, in what time will they cross other travelling in opposite direction?