\(\frac{{dm}}{{dt}} = \)Mass fired per second by the gun
\(\frac{{dm}}{{dt}}\)= Mass of bullet \((mB) ×\) Bullets fired per sec \((N)\)
Maximum force that man can exert \(F = u\;\left( {\frac{{dm}}{{dt}}} \right)\)
\(\therefore F = u \times {m_B} \times N\)
\(⇒ N = \frac{F}{{{m_B} \times u}} = \frac{{144}}{{40 \times {{10}^{ - 3}} \times 1200}} = 3\)