two bullet trains of equal lengths take 10 seconds and 30 seconds respectively to cross a telegraph post. if the length of each bullet train be 120 metres, in what time ( in seconds ) will they cross each other travelling in opposite direction?  Choices:
A) 13 sec .
B) 14 sec .
C) 12 sec .
D) 15 sec .
E) 19 sec .
<gadget id="calculator">120 + 120</gadget>
<output>240</output>

<gadget id="calculator">120 / 10</gadget>
<output>12</output>

<gadget id="calculator">120 / 30</gadget>
<output>4</output>

<gadget id="calculator">12 + 4</gadget>
<output>16</output>

<gadget id="calculator">240 / 16</gadget>
<output>15</output>

<result>D</result>
two bullet trains of equal lengths take 10 seconds and 30 seconds respectively to cross a telegraph post. if the length of each bullet train be 120 metres, in what time ( in seconds ) will they cross each other travelling in opposite direction?
divide(add(120, 120), add(speed(120, 10), speed(120, 30)))
"speed of the first bullet train = 120 / 10 m / sec = 12 m / sec . speed of the second bullet train = 120 / 30 m / sec = 4 m / sec . relative speed = ( 12 + 4 ) = 16 m / sec . required time = ( 120 + 120 ) / 16 sec = 15 sec . d"