Hi,

U remember that though the shell breaks into two, the centre of mass doesnot deviate from its path (very imp statement)

Suppose, r be the distance of landing of cannon here,r=2  )

For the 2m mass,  2mv=0+mv'   (conserving momentum)...............(1)

 For the shell,  r/2=vt (since it falls fron top most point)

From (1), u have vel of the other block to be 2v

 so, its dist from the topmost point is 2vt=2(r/2)=r

HENCE, ITS DIST FROM STARTING POINT WILL BE r/2+r = 1.5r

                                                   hope its clear.....kghedriu