a) when do you use nCr and nPr (I think i know but i want to make sure)

b) if i am flipping a coin what is the probability of me getting head exactly 6 of the 8 times (I think you use nCr p^(something)(1-p)^(something)

c) if i am flipping a coin what is the probability of me getting heads at least 6 of the 8 times?