Half-life of a radioactive substance $A$ is two times the half-life of another radioactive substance $B$. Initially the number of nuclei of $A$ and $B$ are $N_A$ and $N_B$ respectively. After three half-lives of $A$, the number of nuclei of both are equal. Then $N_A / N_B$ is

The ratio of the relative strengths of strong and weak nuclear forces is

In a hypothetical Bohr hydrogen atom, if the mass of the electron is doubled then the energy of the electron in the first orbit is

The half-life period of element $X$ is same as the mean life time of element $Y$. Assume initially $X$ and $Y$ have same number of atoms. Then