$*5\mathrm{.}2\mathrm{.}4$ FeS can be roasted in $O_2$ to form FeO :

$2$FeS$+3O_{2}2$FeO$+2S{O}_2$

If the slag $($solid product$)$ contains $80\%$FeO and $20\%$FeS, and the exit gas is $100\%S{O}_2,$ determine the extent of reaction and the initial number of moles of FeS. Use $100g$ or $100lb$ as the basis.