A natural gas$-$fired industrial boiler operates with an oxygen concentration of $4$ mole percen $($Xor$=0\mathrm{.}04)$ in the flue gases $($burned gasses in the products of the combustion$).$ Determine $($a$)$ the operating air$-$fuel ratio and $($b$)$ the equivalence ratio. Treat the natural gas as methane.

Assume no dissociation. Consider MW$.=28\mathrm{.}87$ and MWs $=16\mathrm{.}04$ AF$=($A$/$F$)/($A$/$F$)$