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**JeanetteR** 3. The life of one species of poplar tree is normally distributed with mean 20 years and standard deviation 2.3 years.

(a) If one tree is selected at random, what is the probability that it will live 21 years or more?

I used Z = x - u/o to find a 67% probability

(b) If nine trees are selected at random, what is the probability that the sample mean life of the nine trees will be 21 years or more?

What formula do I need to use for this part? I am working on a bunch of take-home problems and my head is swimming afer 6 hours! Thanks!