A grandfather clock takes 30 seconds to strike six. How long does it take to strike twelve?
If there are 6 bongs, there are 5 spaces between them, which take 30 seconds to sound. Thus each space between bongs is 6 seconds long.
When there are 12 bongs, there are 11 spaces between them. If each is 6 seconds long, then the total time is 66 seconds.