In a particular retail clothing store, approximately 25% of potential customers who walk into the store make a purchase.

In a random sample of 20 customers that walked into the store,
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a. what is the probability that exactly 10 of the customers make purchases?
b. what is the probability that at least 2 of the customers make purchases?
c. how many of the customers are expected to make purchases?
d. what is the standard deviation of the number of customers who make purchases?