Half lives of two radioactive nuclei A and B are 10 minutes and 20 minutes, respectively. If, initially a sample has equal number of nuclei, then after 60 minutes, the ratio of decayed numbers of nuclei A and B will be:

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Correct answer: D. 9: 8

Half-life of A : t _ (1)/(2) ( A ) = 10 minutes Number of half-lives in 60 minutes are: (60)/(10) = 6 ∴ Number of decayed nuclei of A N _ A = N _ o - N _ o 2 ^ 6 = (63)/(64) N _ o Similarly, Half-life of B : t _ (1)/(2) ( B ) = 20 minutes Number of half-lives of B in 60 minutes are: (60)/(20) = 3 ∴ Number of decayed nuclei of B N _ B = N _ o - N _ o 2 ^ 3 = (7)/(8) N _ o ∴ N _ A : N _ B = (63)/(64): (7)/(8) = 9: 8 = 9: 8

Half lives of two radioactive nuclei A and B are 10 minutes and 20...

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