A. 24 years
B. 20 years
C. 10 years
D. 2 years

Correct Answer:

Option B – 20 years

Explanation

1/64 is the usual fraction left of any radio active material after 4 half-lives, since 1/64 = (1/2)6
This implies that 6 half-lives have elapsed.
Thus 6 half lives = 120 years
∴ 1 half life = 120/6 = 20 years

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