Consider two planets with uniform mass distributions. the mass density and the radius of planet 1 are rho1 and r1, respectively, and those of planet 2 are rho2 and r2. what is the ratio of their masses?

Explanation:

The mass density of an object with uniform mass distribution its defined as

.

So, if we know the volume, and, we can obtain the mass of the object:

.

Now, we can take the planets as spheres, of course, this is only an approximation, but good enough for us. The volume of a sphere of radius r its:

So, for our planets, the mass its given by:

,

so

Now, we can take the ratio:

Now, we can just cancel the .

+15-35=-20

going in opposite directions