A solid of variable density is in the form of a sphere (x^2 + y^2 + z^2 < a^2, where a>0. The density at the point (x,y,z) is z^2. Use triple integration to find the mass of the solid.

Solution given:

**Spoiler:**

I understand most of the question (basically from line 2 down), but what I can't work is where the 1/9th came from. Any help please? :l