Suppose a multiple-choice test has 10 questions each with 5 choices. If a student guesses randomly. What is the probability that they get an B or better. ( they get at least a 80% on the exam) make sure to declare a binomial random variable and show all work

Answer 1

get 8 , 9 or 10 right

p right = .2
p not right = .8

binomial distribution

p 8 out of 10 = C(10,8)* .2^8 *.8^2
= 45 * 2.56*10^-6 * .64 = 73.7*10^-6

p 9 out of 10 = C(10,9)*.2^9 * .8^1
= 10 * 5.12*10^-7 * .8 = 4.096*10^-6

p 10 out of 10 = 1*.2^10 = .1024*10^-6

add 78 * 10^-6
78 out of a million, forget about it