Force of attraction between two spheres is 3.75 x 10^-5 dyne when the spheres are placed 18 cm apart. What is force of attraction when placed 52 cm apart?

F = k (1/d^2) , where F is the force, d is the distance between them , and k is a constant

for the given:
3.75x10^-5 = k/18^2
k = .01215

so F = .01215/d^2
when d = 52
F = .01215/2704 = appr 4.50x10^-6

or

F1/F2 = (d2/d1)^2
F1/3.75x10^-5 = 18^2/52^2
F1 = 4.50x10^-6