Explanation

(b) From Newton’s second law
$F = n \bullet \left( \frac{\mathrm{\Delta}p}{\mathrm{\Delta}t} \right)$
Here, $F = Force,\ n = number\ of\ bullets\ fired\ per\ second.$
$\frac{\mathrm{\Delta}p}{\mathrm{\Delta}t} = rate\ of\ change\ of\ momentum\ of\ one\ bullet.$
$\Longrightarrow F = n\left\lbrack \frac{mv - 0}{\mathrm{\Delta}t} \right\rbrack$
$\Longrightarrow F = n \times \frac{mv}{\mathrm{\Delta}t}$
$Hence,\ \ F = 144\ N,m = 40g = 40 \times 10^{3}kg$
$and\ v = 1200\ {ms}^{- 1},\ \mathrm{\Delta}t = 1\ s.$
$\therefore 144 = n \times \frac{40 \times 10^{- 3} \times 1200}{1}$
$orn = \frac{144}{4 \times 12} \Longrightarrow n = 3$