Question

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 between them. Express this fact as a variation using c as a constant. Use M1 and M2 for the weights of the two bodies.

Answer 1

This can be built up from a number of pieces.

F = M1M2 gives us joint variation with the weights (masses, really).
F = 1/r^2 gives us inverse variation with the square of the distance

Your job is to mash those together (in addition to the constant c) to give a formula that expresses the whole enchilada.