As the first part comes to the rest means, then it will fall under gravity directly.
First of all maximum height,
H=u²sin²θg
H=400×(12)2×10
H=10m.
As initially the full projectile was supposed to land at A but due to explosion first part land at A', so to make the centre of mass lies at A the second part should lie at B.
f they don't break (explode) range OA
OA=u²sin2θg
OA=400×sin45×210
OA=400×sin90°10
OA=40m.
Therefore ,
AA′=OA2
AA′=20m.
As the first part is at 20 m from A in left side being equal mass the second part will be at 20 m right to A.
From gun it is OB=40+20m=60m.
so gun does the other fragment land at 60m.