The half-lives of radioactive sample are `30` years and `60` years for two decay processes. If the sample decays by both the processes simultaneously. The time after which, only one-fourth of the sample will remain is

Options

A. `10` years
B. `20` year
C. `40` year
D. `60` year

Answer

C

Explanation

`lambda=lambda_(a)+lambda_(2)or (1)/(t)-(1)/(t_(1))+(1)/(t_(2))`, where t is `T_(1//2)`
`T_(1//2)=20` years