c
Number of moles of first gas $=\frac{n_{1}}{N_{A}}$
Number of moles of second gas $=\frac{n_{2}}{N_{A}}$
Number of moles of third gas $=\frac{n_{3}}{N_{A}}$
If there is no loss of energy then
$\mathrm{P}_{1} \mathrm{V}_{1}+\mathrm{P}_{2} \mathrm{V}_{2}+\mathrm{P}_{3} \mathrm{V}_{3}=\mathrm{PV}$
$\frac{n_{1}}{N_{A}} R T_{1}+\frac{n_{2}}{N_{A}} R T_{2}+\frac{n_{3}}{N_{A}} R T_{3}$
$=\frac{n_{1}+n_{2}+n_{3}}{N_{A}} R T_{m i x} \quad T_{\operatorname{mix}}=\frac{n_{1} T_{1}+n_{2} T_{2}+n_{3} T_{3}}{n_{1}+n_{2}+n_{3}}$