11. A manufacturing firm orders computer chips from three different companies: 10% from Company A; 20% from Company B; and 70% from Company C. Some of the computer chips that are ordered are defective: 4% from Company A are defective; 2% from Company B are defective; and 0.5% from Company C are defective. A worker at the manufacturing firm discovers that a randomly selected computer chip is defective. What is the probability that the computer chip came from Company B? Show your work.

13. The National Association of Retailers reports that 62% of all purchases are now made by credit card; you think this is true at your store as well. On a typical day you make 20 sales.
Explain why your sales can be considered Bernoulli trials.
What is the probability that your 4th customer is the first one who uses a credit card?
Let X represent the number of customers who use a credit card on a typical day. What is the probability model for X? Specify the model (name and parameters) and tell the mean and standard deviation.
What is the probability that on a typical day at least half of your customers use a credit card?