If the gravitational force between objects of equal mass is 5.0*10^-9 n when the object are 12 m a part. what is the mass of each object?

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)