We would need to assume that each of those events was spread out over the entirety of the 20 years. All 5 occasions could have happened between 2003 and 2007, a

We would need to assume that each of those events was spread out over the entirety of the 20 years. All 5 occasions could have happened between 2003 and 2007, and fifteen years passed before the next. Each time the cracks were repaired. Without a true timeline of events, its hard to say for sure.