The 'roasting' of $100.0 \mathrm{g}$ of a copper ore yielded $71.8 g$ pure copper. If the ore is composed of ${\mathrm{Cu}}_2\mathrm{S}$ and CuS with $4.5\%$ inert impurity, The percent of ${\mathrm{Cu}}_2\mathrm{S}$ in the ore is ab.c, then (a+b) - c value is 

${\mathrm{Cu}}_2\mathrm{S}+{\mathrm{O}}_2⟶2\mathrm{Cu}+{\mathrm{SO}}_2$ and $\mathrm{CuS}+{\mathrm{O}}_2⟶\mathrm{Cu}+{\mathrm{SO}}_2$

[Atomic masses $\mathrm{Cu} = 63.5, \mathrm{S} = 32$]