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A radioactive substance has a half life of 20 days. What fra...

A radioactive substance has a half life of 20 days. What fraction of the original radioactive nuclei will remain after 80 days?

A.

132

B.

116

C.

18

D.

14

Correct answer is B

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

(12)4=116