Al walks down to the bottom of an escalator that is moving up and he counts 150 steps. His friend Bob, walks up to the top of the escalator and counts 75 steps. If Al's speed of walking (in steps per minutes) is tree times of Bob's speed, how many steps are visible on the escalator at any given time?(Assume that this number is constant).

(第五届AIME1987 第10题)