A nursery advertises that it has 10 elm trees for sale. Unknown to the nursery, 3 of

Question:

A nursery advertises that it has 10 elm trees for sale. Unknown to the nursery, 3 of the trees have already been infected with Dutch elm disease and will die within a year.
a. If a buyer purchases two trees, what is the probability that both trees will be healthy?
b. Refer to part a. What is the probability that at least 1 of the trees is infected?