Internal forces do not affect the motion of the centre of mass, the centre of mass hits the ground at a position where the original projectile would have landed. The range of the original projectile is
`x_(CM)=(2u^(2)sinthetacostheta)/g=(2xx10^(4)xx3/5xx4/5)/10 m=960m`
The centre of mass will hit the ground at this position. As the smaller block comes to rest after breaking, it falls down vertically and hits the ground at half of the range i.e.,at `x=480m`. If the heavier block hits the ground at `x_(2)` then
`x_(CM)=(m_(1)x_(1)+m_(2)x_(2))/(m_(1)+m_(2))implies960((m)(480)+(3m)(x_(2)))/((m+3m))`
`:. x_(2)=1120m`