Two identical 9. 60-g metal spheres (small enough to be treated as particles) are hung from separate \(300-\mathrm{mm}\) strings attached to the same nail in a ceiling. Surplus electrons are added to each sphere, and then the spheres are brought in contact with each other and released. Their equilibrium position is such that each string makes a \(13.0^{\circ}\) angle with the vertical. How many surplus electrons are on each sphere?