I have a test tomorrow, and I am not able to figure out how to do problems of this type:

1. In a paint store, there are x white paint cans of Brand A; y white paint cans of brand B; and z white paint cans of brand C. If 3 customers bought 4 white paint cans, what is the probability that they bought 1 can of brand A, 1 can of brand B and 2 cans of brand C.

2. Also, how do you find the same probability (problem 1) using probability distribution functions?

3. In a normal distribution with 15, 22% score more than 17. What is the standard deviation of this distribution? (maximum is not given and in previous problems, the mean and standard deviation were given. We calculated the z-score and used a table to find the corresponding probability).

I would really appreciate any help - I just cannot understand these problems.

Thanks

-sri