A radioactive element which can decay by two processes, has half-life $t_1$ for first process and half-life $t_2$ for second process. Let $\langle t\rangle$ be the effective average-life of this element. Which of the following is correct?

In atomic scale the weakest force in nature is

The wavelength of a spectral line emitted by hydrogen atom in the Balmer series is $\frac{16}{3 R}$

( $R$ is Rydberg constant). What is the value of the principal quantum number of the state from which the transition takes place?

The half-life of a radioactive sample is 5 s . If the initial mass of the sample is 60 g , then the time required to reduce the sample to 7.5 g is