Center of Mass

As disc is floating in air without falling, force by bullets balances gravitational force.
⇒‌‌(‌

N∆p

∆t

)=mdg
Here, N= number of bullets fired ‌

∆p

∆t

= Rate of change of one bullet md= mass of disc
Hence, bullets rebound with same speed, momentum change for each bullet =m(v−(−v))=2mv
∴‌‌‌

N

t

⋅2mbv=mdg
Substituting given values,
‌10×2×0.05×v=0.2×10
‌∴‌‌v=‌

0.2×10

10×2×0.05

=2m∕s