A radioactive substance of mass 768g has a half life of 3 years. After how many years does this substance leave only 6g undecayed?
21
18
15
12
Correct answer is A
\(768 \to 384 \to 192 \to 96 \to 48 \to 24 \to 12 \to 6\) 7 x 3 = 21yrs
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