Count rate of a radioactive substance is 400 per minute. After some time the count rate is 123 per minute. If half life of the element is 2 hours, then number of nuclei decayed in this interval is

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64, 625

20, 875

32, 120

47, 965

answer is D.

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Detailed Solution

R1=λN1 and R2=λN2∴ λN1−λN2=R1−R2⇒ln2TN1−N2=R1−R2⇒N1−N2=TR1−R2ln2=2×60×400−1230.693⇒N1−N2=47,965

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