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=‌
