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} )
Correct Answer: Option B
Explanation
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} )