To get out of the railway station near Quiz Master Shop Towers there is an escalator that carries passengers from the platform to road level in 30 seconds. Most people simply stand there and travel up without walking, but it is quicker if you walk as well, of course.
One of our colleagues always walks and, being so minded, timed the ascent. Walking reduced the journey time from 30 seconds to 12 seconds.
Unfortunately the escalator was broken yesterday and everyone had to walk up the stationary steps.
How long did our colleague take to walk up the static escalator?
In each second the escalator on its own moves 1/30th of its total length.
When our colleague walked they travelled 1/12th of the escalator's length in each second.
So our colleague contributed 1/12th minus 1/30th of the journey each second, meaning they moved 1/20th of the escalator's length each second.
The journey took 20 seconds.