For wavelength of visible radiation of Hydrogen spectrum, Balmer gave an equation as $\lambda = \dfrac{xm^2}{m^2-4}$, where m is the integer value. The value of x in terms of Rydberg's constant R is

The orbital magnetic moment $(m_{orb})$ of a revolving electron around the nucleus varies with the principal quantum number ($n$) as

The half-life of the isotope ${}_{11}\text{Na}^{24}$ is 15 hr. How much time does it take for $\left(\dfrac{7}{8}\right)^{\text{th}}$ of a sample of this isotope to decay ?