Of all customers purchasing automatic garage-door openers, 75% purchase a chain-driven model. Let X = the number

Question:

Of all customers purchasing automatic garage-door openers, 75% purchase a chain-driven model. Let X = the number among the next 15 purchasers who select the chain-driven model.
a. What is the pmf of X?
b. Compute P(X > 10).
c. Compute P(6 < X < 10).
d. Compute µ and σ2.
e. If the store currently has in stock 10 chain-driven models and 8 shaft-driven models, what is the probability that the requests of these 15 customers can all be met from existing stock?