Need major help on this one. Sooo confused...

Question

Binomial distribution to solve the problems. There is a 60 percent probability that a store will sell one of Red bags to any customer who comes into the shop and views the bags. Out of the next 7 people who come into the shop, what is the probability that 3 people will buy a bag? What is the probability that more than four people will buy the bag?

Answer 1

The probability of three purchases after seven customer visits is
P(3) = (0.6)^3*(0.4)^4*C(7,3)
where C(7,3) is the binomial coefficient
C(7,3) = 7!/(4!*3!) = 70

I get 0.387 for the answer.

For P(n>4), add P(5), P(6) and P(7)

For an explanation of how the probability relates to the binomial coefficient, see
http://en.wikipedia.org/wiki/Binomial_coefficient

Answer 2

How do you get 70. When I calculate the formula I get 35.

Answer 3

7!/(4!*3!) = 5*6*7/(1*2*3)= 210/6 = 35
You are right