Step 1: Newton's Second Law,
From Newton's second law
F=dPdt ....(1)
Where, P=mv is the momentum
Step 2: Total bullets fired per sec
Let n be the total number of bullets fired
The force exerted by man must balance the change in momentum of the bullets per second.
∴P=nmv ; m and v are constant (Given)
⇒F=ddt(nmv)
⇒ dndt=Fmv=144(N)40×10−3(kg)×1200(m/s)=3
Hence total number of bullets fired per second i.e., dndt=3.
Hence, Option B is correct