Question 1077781
interval is 1.96*s/sqrt(n).  I will use z, as I suspect n is large.  If not, I will iterate with t.
1.96*17.7/sqrt(n)=4
n=75.22 when squaring and rearranging.
Will use t=2
[2*17.7/4]^2=78.32 or 79. 
This has a t=1.99
n=78
with 2 books, this will be a much higher sample size
313.29, round to 314, and z is used here.
For the  last, I am going to assume 99% is meant.
use 2.576 for z
129.93 or 130 books.