The gravitational force is
F = G (one mass) (the other mass) / (distance between them)²
G = the universal gravitational constant
= 6.67 x 10⁻¹¹ newton-meter²/kg²
If the masses are equal, then F = G m²/R²
Divide each side by 'G': F/G = m²/R²
Multiply each side by R² : m² = F R²/ G
Take the square root of each side: m = √(FR²/G)
Mass = √(5 x 10⁻⁹ x 144/ 6.67 x 10⁻¹¹)
= √(500 x 144/ 6.67)
= 103.9 kilogram (rounded)