7 books with a cost of 28$ then a 2$ charge which will put him right at 30
F(x)=10-16/x
f'(x)=16/x^2
f'(c)=16/c^2
f(8)=10-16/8=10-2=8
f(2)=10-16/2=10-8=2

4∈[2,8] is the required point.
The formula for the amount A in an account with principal P and interest rate r compounded annually for t years is
... A(t) = P(1+r)^t
You want to find A when P=400, r=0.05, and t=3. Substituting those values gives you
... A(3) = 400·(1 +0.05)³
The appropriate choice is
... A. A(3) = 400·(1 +0.05)³