The mean lives of a radioactive sample are 30 years and 60 years for - emission and
-emission
respectively. If the sample decays both by -emission and
-emission simultaneously, the time after which,
only one-fourth of the sample remains is:
14 years
20 years
28 years
45 years
One – fourth of sample will remain after 2 half life = 27.72 years.
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