Work = 2.4 J    Time = 1 minute = 60 second
$\frac{\text{ Work }}{\text{ time }}=\frac{2.4}{60}=0.04\text{ watt, Power }=2\text{ watt }$
    Number of times lung expand, 
$\mathrm{n}=\frac{\text{ power }}{\text{ work per unit time }}$
$\mathrm{n}=\frac{2}{0.04}=50\text{ times. }$