A radioactive substance of mass 768g has a half life of 3 years. After how many years does this substance leave only 6g undecayed?
A
21
B
18
C
15
D
12
Verified Correct Answer: A.
Physical Principles & Numerical Solution
\(768 \to 384 \to 192 \to 96 \to 48 \to 24 \to 12 \to 6\)
7 x 3 = 21yrs
Source: Myschool Past Questions Archive (Record #34703)