1. A class is given a list of 20 study problems from which ten will be part of an upcoming exam. If a givent student knows how to solve 15 of the problems, find the probability that the student will be able to answer at least nine question on the exam.
2. Four letters and envelopes are addressed to four different people. If the letters are randomly inserted into the envelopes, what is the probability that (a)exactly one is inserted in the correct envelope and (b) at least one is inserted in the correct envelope?
3. A shipment of 12 microwave ovens contains three defective units. A vending company has ordered four of these units, and because all are packaged identically, the selection will be random. What is the probablity that a) all four units are good? b) exactly two units are good? c) at least two units are good?
4. A sales representatives make sales at approximately one-fifth of all calls. If, on a given day, the representative contacts six potential clients, what is the probability that a sale will be made with (a) all six contacts, (b) none of the contacts, and (c) at least one contact?