two goods trains each 500 m long are running in opposite directions on parallel tracks . their speeds are 30 km / hr and 90 km / hr respectively . find the time taken by the slower train to pass the driver of the faster one ?
"relative speed = 90 + 30 = 120 km / hr . 120 * 5 / 18 = 100 / 3 m / sec . distance covered = 500 + 500 = 1000 m . required time = 1000 * 3 / 100 = 30 sec . answer : b"
add(30, 90)