The radioactivity of a given sample of whisky due to tritium (half life 12.3 years) was found to be only 3% of that measured in a recently purchased bottle marked "7 years old". The sample must have been prepared about

220 years back

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300 years back

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400 years back

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70 years back

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Solution

The correct option is C 70 years back
After one half life period, the activity of Tritium becomes $50 %$
After $2$ half life period $25 %$
After $3$ half life period $12.5 %$
After $4$ half life period $6.25 %$
After $5$ half life period $3.12 % \approx 3 %$
It is $5 \times 12.5 y e a r s + 7 y e a r s$ i.e. approximately $70 y e a r s$ only.

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