A hydrogen atom falls from $n$th higher energy orbit to first energy orbit $(n=1)$. The energy released is equal to 12.75 eV . The $n$th orbit is

The decrease in each day in the uranium mass of the material in a uranium reactor operating at a power of 12 MW is (Energy released in one ${ }_{92} \mathrm{U}^{235}$ fission is about 200 MeV )

If the binding energy of the electron in a hydrogen atom is 13.6 eV . Then, energy required to remove electron from first excited state of $\mathrm{Li}^{2+}$ is

A mixture consists of two radioactive materials $A_1$ and $A_2$ with half lives of 20 s and 10 s respectively. Initially, the mixture has 40 g of $A_1$ and 160 g of $A_2$. The amount of the two in the mixture will become equal after