A) 40
B) 60
C) 80
D) 100
Correct Answer: C
Solution :
[c] For this substance 7 days correspond to \[3.5\] half- lives. Over 3 half- lifes the sample reduces to \[\frac{1}{{{2}^{3}}}=\frac{1}{8}\] of its initial mass. After 4 half- lifes, the sample has only \[\frac{1}{{{2}^{4}}}=\frac{1}{16}\]of its initial mass. Hence, after \[3.5\] half- lives the sample must contain somewhere between \[1/8\] and \[1/16\] of its initial mass. Hence 5 g is somewhere between 1/8 and \[1/16\] of the initial mass. So, the initial mass is somewhere between \[8\times 5=40g\] and \[16\times 5=80g.\]
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