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 unit time) is three times Bob's walking speed, how many steps are visible on the escalator at a given time? (Assume that this value is constant.)
Let the total number of steps be $x$, the speed of the escalator be $e$ and the speed of Bob be $b$.
In the time it took Bob to climb up the escalator he saw 75 steps and also climbed the entire escalator.  Thus the contribution of the escalator must have been an additional $x - 75$ steps.  Since Bob and the escalator were both moving at a constant speed over the time it took Bob to climb, the [ratio](https://artofproblemsolving.com/wiki/index.php/Ratio) of their distances covered is the same as the ratio of their speeds, so $\frac{e}{b} = \frac{x - 75}{75}$.  
Similarly, in the time it took Al to walk down the escalator he saw 150 steps, so the escalator must have moved $150 - x$ steps in that time.  Thus $\frac{e}{3b} = \frac{150 - x}{150}$ or $\frac{e}{b} = \frac{150 - x}{50}$.
Equating the two values of $\frac{e}{b}$ we have $\frac{x - 75}{75} = \frac{150 - x}{50}$ and so $2x - 150 = 450 - 3x$ and $5x = 600$ and $x = \boxed{120}$, the answer.