# a problem with normal distribution

I have some problems with this

The annual rainfall (in inches) in a certain region is normally distributed with μ = 40 and σ = 4. What is the probability that, starting with this year, it will take over 10 years before a year occurs having a rainfall of over 50 inches? What assumptions are you making?

Let X be a normal random variable with μ = 40 and σ = 4 that represents the annual rainfall. $$P(X>= 50)=P((X-\mu )/\sigma \ge (50-10)/4 =2,5)=1-\Phi (2,5)=0.9938$$

Let Y be a geometric random variable with parameter $$p=0,9938$$ The probability that it will take over 10 years before a year occurs having a rainfall of over 50 inches would be $$P(Y=11)=0,9938^{10}*(1-0,9938)$$ because at the eleventh year we will have surely a rainfall of over 50 inches. But the solution on the book is $$0,9938^{10}$$ only.

Why?

I'm assuming that each event is independent from the others.

• "Over 10 years before it rains" is the event $Y \geq 11$. Dec 29 '16 at 11:45