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