A radioactive substance has a half life of 20 days. What fraction of the original radioactive nuclei will remain after 80 days?
A
\( \frac{1}{32} \)
B
\( \frac{1}{16} \)
C
\( \frac{1}{8} \)
D
\( \frac{1}{4} \)
Verified Correct Answer: B.
Physical Principles & Numerical Solution
80 days = 4 half lives (i.e 4 x 20dqs = 80dq)
But the fraction always left by any radioactive material after 4 half lives is
\( \left ( \frac{1}{2} \right)^4 = \frac{1}{16} \)
Source: Myschool Past Questions Archive (Record #41396)