Acceleration of an electron in the first Bohr's orbit is proportional to $\mathrm{m}=$ mass of electron, $\mathrm{r}=$ radius of the orbit, $\mathrm{h}=$ Planck's constant)

In the given reaction

$${ }_z \mathrm{X}^A \rightarrow{ }_{z+1} \mathrm{Y}^A \rightarrow{ }_{z-1} \mathrm{~K}^{A-4} \rightarrow{ }_{z-1} \mathrm{~K}^{\mathrm{A}-4}$$

radioactive radiations are emitted in the sequence

A radioactive substance has half-life of 60 minute. During 3 hour, the amount of substance decayed would be

The ratio of the areas of the electron orbits for the second excited state to the first excited state for the hydrogen atom is