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A radioactive nucleus A with a half life T, decays into a nucleus B. At t = 0, there is no nucleus B. At sometime t, the ratio of the number of B to that of A is 0.3. Then, t is given by -

1. t = 2 T 3 . 1 log 2 log
2. t = T 2 log 3 . 1 log
3. t = T log (1.3)
4. t = ) 3 . 1 log( T

Correct answer: t = T 2 log 3 . 1 log

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