Simple Question 20 Poem by Kim Barney

(4) When the clock strikes seven, there are six five-second gaps of time between the bongs, so that means it takes thirty seconds for the clock to strike seven.

(3) When the clock strikes five, there are four gaps between the bongs. Since those four gaps take twenty seconds, each gap is five seconds long, i.e. there are five seconds between each bong.

(2) You have to consider the 'gaps of time' BETWEEN the 'bongs' rather than the 'bongs' themselves.

Time has expired on this question and it's time to give the solution: Exactly thirty seconds. A more detailed explanation will be given in a separate comment, since it would be too long in this one.