Answer
a) .0514
b) .815
c) The survey is likely not valid.
Work Step by Step
a) We find that the probability is:
$\frac{16!}{(12!)(4!)}(.9^{12})(.1^4)=.0514$
b) The probability is:
$1-(.9^{16})=.815$
c) The survey is likely not valid, for when it is a voluntary sample, mainly the most extreme opinions, not the general opinion, will be reflected.
