Two trains of equal lengths take 10 sec and 20 sec respectively to cross a telegraph post. If the length of each train be 120 m, in what time will they cross other travelling in opposite direction?	Select: A) 16 sec B) 13 sec C) 17 sec D) 21 sec E) 23 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= 120/20= 
<gadget id="calculator">120/20</gadget>
<output>6</output>
6 m/sec.   Relative speed= 12 + 6= 
<gadget id="calculator">12 + 6</gadget>
<output>18</output>
18 m/sec.   Required time= (120 + 120)/18 = 13 sec.   Answer: 13 sec
<result>B</result>
Two trains of equal lengths take 10 sec and 20 sec respectively to cross a telegraph post. If the length of each train be 120 m, in what time will they cross other travelling in opposite direction?