2012 AMC 10B Problem 13

Let $d$ be the escalator’s length, and let $c$ represent Clea’s speed walking down the non-operational escalator. Similarly, let $e$ represent Clea’s speed standing still on the operational escalator. Then, as speed is equal to distance over time, we know that $c = \dfrac d{60}$ and $$c+e = \dfrac d{24}.$$ Therefore, $$e = \dfrac d{24}-\dfrac d{60} = \dfrac d{40},$$ meaning that Clea takes $40$ seconds to descend the escalator by simply standing still. Thus, the correct answer is B .