20
There are 10 feet of posts on each side. That means there are 40 feet of fencing in total. If you divide that by two (to represent posts being 2 feet apart) then you get 20. There are 20 posts needed.
A because I know I’m right because I studied a little bit and I got to get it right
The next term is double the previous term, so that the
-th term is given recursively by

This rule tells us that



and so on, with the explicit rule

for
.
If 512 is the
-th term in the sequence, then

77(7) = 539, the total points of all her tests
539 - 18 + 81 = correct total points = 602
602/7 = 86 which is the new average