I've came up with a math problem after applying weed killer to my lawn, and the directions say it should be watered a day or two later. So I looked up the weather forecast, and it said that the chance of rain for the following three days is 50%, 40%, and 30%, so what is the chance that it will rain at some point over the entire 3 day period? It seems like maybe you could average the three percentages, but I don't think that works, because with tomorrow alone there is a 50% chance, so for the three day period there has to be a minimum of a 50%. Day two is less than 50%, but it seems like adding another day, even if it is a lower chance, should actually increase the chances over all. Is there an answer here or not?

Put another way if there is a 10% chance for 100 days in a row, wouldn't a chance of rain over that many days approach a certainty?