The force f of attraction between two bodies varies jointly as the weights of the two bodies and inversely as the square of the distance d between them. express this fact as a variation using c as a constant. use m1 and m2 for the weights of the two bodies.

Since we are to express the given conditions as a joint variation, we employ the use of k as our constant proportionality which we will denote in this item as k. 
 From the sentence,
                            F = kM₁M₂/d²
as we can see those in the denominator as those that vary inversely to the given parameter. 

The answer for this question is 80