The probability that a randomly selected box of a certain type of cereal has a particular prize

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The probability that a randomly selected box of a certain type of cereal has a particular prize is .2. Suppose you purchase box after box until you have obtained two of these prizes.
a. What is the probability that you purchase x boxes that do not have the desired prize?
b. What is the probability that you purchase four boxes?
c. What is the probability that you purchase at most four boxes?
d. How many boxes without the desired prize do you expect to purchase? How many boxes do you expect to purchase?
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