two bullet trains of equal lengths take 10 seconds and 20 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 ?
"speed of the first bullet train = 120 / 10 m / sec = 12 m / sec . speed of the second bullet train = 120 / 20 m / sec = 6 m / sec . relative speed = ( 12 + 6 ) = 18 m / sec . required time = ( 120 + 120 ) / 18 sec = 13.3 sec . c"
divide(add(120, 120), add(speed(120, 10), speed(120, 20)))