Boris is playing on the mall escalators. One escalator goes up, one goes down, and one is out of service; otherwise, they're all identical. The up and down escalators go at the same speed. You can assume that Boris always runs at the same speed.
Boris can run up the up escalator in seconds. He can run up the down escalator in seconds. How long does it take him to run up the out-of-service escalator? Explain your answer.