Answer
2 hours.
Work Step by Step
The amount of radioactive material remaining after eight hours is one-sixteenth of the original.
After one half-life, we are down to one-half the original amount.
After two half-lives, we are down to one-quarter the original amount.
After three half-lives, we are down to one-eighth the original amount.
After four half-lives, we are down to one-sixteenth the original amount.
There have been four elapsed half-life periods in 8 hours. (8 hours)/4 = 2 hours.
