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 6 seconds. he can run up the down escalator in 30 seconds. how long does it take him to run up the out-of-service escalator? explain your answer.

hint(s):

let e be the moving escalators' speed and let b be boris's running speed. in terms of e and b, what speeds can you learn from the problem? try expressing these speeds in flights per minute (a flight is one trip from the bottom to the top).