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JEE Main levelSingle correct optionDifficulty 2 of 3as175

Samples A and B contain equal numbers of atoms initially. The half-lives are $10 min$ for A and $20 min$ for B. The ratio (activity of A) : (activity of B) after $40 min$ is

A1 : 2
B2 : 1
C1 : 1
D1 : 4

Answer: A

After 40 min A has

N_{0} /16

atoms and B has

N_{0} /4

atoms. Since

λ_{A} = 2 λ_{B}

, activity ratio

= (2 \times \frac{1}{16}) / (\frac{1}{4}) = 0.5

, that is 1 : 2.

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