Dry coke composed of $4\%$ ash, $90\%$ carbon and $6\%$ hydrogen by mass is burned in a furnace with dry air. The solid refuse left after combustion contains $10\%$ carbon and $90\%$ ash by mass. The Orsat Analysis of the flue gas gives $13\mathrm{.}9\%$ CO$2,0\mathrm{.}8\%$ CO$,4\mathrm{.}3\%$O$2$ and $81\%$ N$2$ by mole. Calculate $($a$)$ weight of refuse per $100$ g coke $($b$)$ the $\%$ excess air.