Two small spheres each of mass 10 mg are suspended from a point by threads 0.5 m long. They are equally charged and repel each other to a distance of 0.20 m . The charge on each of the sphere is $\frac{\mathrm{a}}{21} \times 10^{-8} \mathrm{C}$. The value of ' a ' will be $\_\_\_\_$ [Given $\mathrm{g}=10 \mathrm{~ms}^{-2}$ ]