In the first collision mu momentum will be imparted to the system. In the second collision when the momentum of (M+m) is in the opposite direction mu momentum of the particle will make its momentum zero. On 13th collision,

Applying momentum conservation
mu=(M+13m)v
⇒v=M+13mmu=15u ⇒M=2m
⇒v=ωA
⇒15u=M+13mK×A
⇒A=151175=31