Binomial Math
posted by Bersy .
From experience, the manager of Kramer's Book Mart knows that 40% of the people who are browsing in the store will make a purchase. What is the probability that among ten people who are browsing in the store, at least five will make a purchase? (Round your answer to four decimal places.)

prob of purchase = .4
prob of no purchase = .6
prob of at least 5 from 10 will make purchase
= prob(5will buy) + prob(6 will buy) + ..+ prob(10 will buy)
....
lots of arithmetic, I will do the prob 6 will buy
= C(10,6) (.4)^6 (.6)^4
= ..
What might be a shorter way is to exclude the cases of none, 1, 2 , 3 or 4 will buy
= 1  (prob(0 will buy + prob(1 will buy ...+prob(4 will buy) 
.2150