If the volume of an object were to double with no change in mass, its density would

HALVED

Explanation:

The formula for density is  density = mass /volume .

We need to double the volume. multiply it by  2 .

But in order to multiply one side of the equation by a number, the same has to be done to the other side.

Multiplying the volume by  2  is the same as multiplying the fraction  mass  volume  by  1/ 2 .

So let's multiply both sides of the equation by  1 /2 :

⇒ 1 /2 × density = 1 /2 × mass /volume

⇒ density /2 = mass /2 × volume

not sure if this what   you'r looking for would be percent composition