This is a geometric sequence of the form:
a(n)=ar^(n-1), a=initial term, r=common ratio, n=term number
The number of books doubles each week so the common ratio is 2 and the first term a is 1 so
a(n)=1*2^(n-1)
a(n)=2^(n-1) so in the sixth week
a(6)=2^5=32
So she will read 32 books in the sixth week.
