How much of a radioactive substance remains after three half-lives?
a) one third
b) one half
c) one quarter
d) one eighth
e) one sixth
After each half-life, the amount of a radioactive substance remaining is halved. So, after the first half-life, half of the substance remains. After the second half-life, half of that remaining half remains, which is one-fourth of the original substance. After the third half-life, half of that remaining one-fourth remains, which is one-eighth of the original substance.
Therefore, the answer is d) one eighth.