Let's say you made a special set of dice. Each one had 100 equal sides, not just six like regular dice. If you roll one of these there is a 1 in 100 chance that you'll hit the side with 1 dot.
If you'd like to know what your odds of getting hit in 5 years you could to roll five of these special dice and look for a 1 on any of them. Since each one has a 99% of not showing a 1, the changce that there are no 1's showing at all is .99 x .99 x.99 x.99 x.99=.951. So you have just over a 95% chance of being safe for five years.
More years, just get more of these special dice. The answer comes out 0.99
n where n is the number of years and the answer is the probability of being safe from flooding. Here's a little table to showing the odds of staying safe for a given number of years:
| Years |
Safety |
| 10 |
90.44% |
| 20 |
81.79% |
| 30 |
73.97% |
| 40 |
66.90% |
| 50 |
60.50% |
| 60 |
54.72% |
| 70 |
49.48% |
| 80 |
44.75% |
| 90 |
40.47% |
| 100 |
36.60% |