Answer:
The gravitational force between two objects is written as:
F = G*m1*m2/r^2
Where:
G = 6.67*10^(-11) m^3/kg*s
m1 and m2 are the masses of the objects.
r is the distance between the objects.
In this case, we know that:
F = 0.0005 N
r = 10m
and because the two spheres are identical, we have that:
m1 = m2 = M
then we have the equation:
0.0005 N = (6.67*10^(-11) m^3/kg*s)*(M^2/(10m)^2)
0.0005 N*(10m)^2/(6.67*10^(-11) m^3/kg*s) = M^2
√( 0.0005 N*(10m)^2/(6.67*10^(-11) m^3/kg*s) ) = M = 27,379.3 kg
The mass of each sphere is 27,379.3 kg