7.

Second line.
with a $n$ dollars --->  with $n$ dollars ?

5th line
a bit ->> a bet

(b)
I wonder if we should use W(x) instead of w(x).
I believe that most generating functions we've seen in this
class are uppercases.


9.


4th statement: 
I'd suggest
"will be chosen a second time" ---> "will be chosen again"
The 'second time' might confuse the student into thinking
it's about the second choice only...


Last statement:
I'm not sure why it's true, if $d$ is the real fraction of defective
wafers.  And since the choices are independent... the result
of the first one should not affect the prob. of the second one?
But I guess it does not affect the problem, because
we can reconsider the solution later...