Explanation:
when a variable depends on the product or quotient of two or more variables, it is a joint variation like in our case here.
F (e.g. gravity) varies directly with the masses of 2 objects (like 2 planets) and varies inversely with the distance to or between these objects.
so, the gravity between 2 planets gets stronger the more mass the planets have, but gets weaker the more distance there is between them.
there is only one constant considered in a joint variation statement. in our case : k