Answer
See below. Cesium-137 has a half-life of 30 years.
Work Step by Step
After one half-life, at t = 30 years, we are down to one-half the original amount.
After two half-lives, at t = 60 years, we are down to one-quarter the original amount.
After three half-lives, at t = 90 years, we are down to one-eighth the original amount.
After four half-lives, at t = 120 years, we are down to one-sixteenth the original amount.
This is discussed on page 625.