I am cursed with a horrible text book. In regards to a normal distribution: when given a mean and standard deviation, I can compute a probability of precipitation in inches with no problem. IE: when asked what is the probability that rainfall in a given area will exceed a given # of inches: I calculate standard score from: (Xi - mean)/standard deviation. I then use this value and a z score table. I then evaluate the probability value and come up with an answer like "in ___ years out of 100, the annual precipitation in the given region should exceed given amount of inches." HELP HERE: I am asked "if conditions continue, the region should receive at least ___ inches in 19 out of 20 years". Am i supposed to work the problem backwards? in short: when only given the # of years out of so many years, and given a mean and standard deviation only, how do i find the amount of precipitation expected? it sounds like the reverse of the above problem, where you're given the precip. level, the mean, and the SD, and asked to find the # of years out of 100.