A gas is contained in a cylinder and expands according to the relation $P V^{1.3}=$ constant. The initial pressure and initial volume of the gas is $30 \mathrm{~atm}$ and $30 \mathrm{~mm}^{3}$ respectively. If the final pressure is $15 \mathrm{~atm}$, then the work done on the face of piston by the pressure force of the gas, is :
(a) $5 \times 10^{i} \mathrm{~J}$
(b) $4.35 \times 10^{4} \mathrm{~J}$
(c) $3 \times 10^{4} \mathrm{~J}$
(d) $4 \times 10^{4} \mathrm{~J}$