The wavelengths of ${\text{K}}_{\alpha }$ $X$-rays of two metals $\mathrm{A}$ and $\mathrm{B}$ are $\frac{4}{1875\mathit{R}}$ and $\frac{1}{675\mathit{R}}$ respectively, where $R$ is Rydberg constant. The number of elements lying between $\mathrm{A}$ and $\mathrm{B}$ according to their atomic numbers is