Question 246478
a-
There are 15 questions.
Each question has 5 possible answers.
5^15 is not correct.
Why are you trying to raise 5 to a power?
Think about what that does. 
let's say there are only two questions with 5 possible answers
so with your method there would be 5*5 possible answers=25
but we know 
the first question has 5 possible answers
the second questions has five possible answers

lets make it only three questions
the first question has 5 possible answers
the second questions has five possible answers
the third question has five possible answers
This should give you a hint to how many answers 15 questions might have.
b-
so what has happened?
8 questions have been reduced to only one possible answer -the correct one.
that leaves 7 with 5 possible answers
so work out how many possible answers there are for those 7 and add 8 to it for the 8 correct answers