Answer
See below.
Work Step by Step
After one half-life, at t = 1 year, we are down to one-half the original amount.
After two half-lives, at t = 2 years, we are down to one-quarter the original amount.
After three half-lives, at t = 3 years, we are down to one-eighth the original amount.
After four half-lives, at t = 4 years, we are down to one-sixteenth the original amount.
This is discussed on page 625.