> On exercise 1.7, will you please explain the question for part b?

The point is to show that your answer to part (a) is much bigger than
10^18.  You can show that by computing the answer to part (a) exactly, but
that's very tedious.  A calculator gives a close value (maybe not exact
because the calculator probably does not have enough digits).  Or you can 
use some easy reasoning to show that your answer to part (a) is very big
by other methods.