Question 668672
With each question having 1 right answer out of 5,  p = 1/5 = .2

Now in order to get an A you must get 5 or more correct. So, basically we are trying to find the P[5 correct] + P[6 correct] + P[7 correct]

P[5 correct]  = (7 choose 5) * (.2)^5 * (.8)^2 because we are choosing 5 that we want to get right out of 7, there's a .2 chance of getting each right (done 5 times) and there's a .8 chance of getting it wrong (done 2 times)

P[6 correct] = (7 choose 6) * (.2)^6 * (.8)^1

P[7 correct] = (7 choose 7) * (.2)^7 * (.8)^0

So P[>= 5 correct] = P[5 correct] + P[6 correct] + P[7 correct] = (7 choose 5) * (.2)^5 *(.8)^2 + (7 choose 6) * (.2)^6 * (.8)^1 + (.2)^7 = {{{highlight(0.0047)}}}