Well, I'm not much of a scientist..... and homework and I have never gotten along very well....
The Kuiper belt, mostly ice, is approximately 1/10 the mass of the earth.
The asteroid belt, mostly rock, is approximately 1/2400 the mass of the earth.
According to Brown, there was a huge explosion on earth a long time ago. This explosion sent lots of rock and water flying into space. If this is the case, then there was plenty of rock and water existing on earth at the time to completely account for all of the mass we find in both the asteroid belt and the Kuiper belt, so I'm not completely sure where you're going with this.