An old-fashioned clock takes 30 seconds to strike six o'clock.
How long will it take to strike twelve o'clock, in seconds?
Assumption: Each strike is instantaneous, and the interval between strikes is constant.
To strike six o'clock, there are 5 intervals.
So, under the assumption that each strike is instantaneous, it will take 30 seconds / 5 intervals, or 6 seconds per interval.
To strike twelve o'clock, there are 11 intervals.
A 6 seconds per interval, this will take 11 x 6 = 66 seconds.