For the following given data, estimate upstream pressure at choke:

Downstream pressure: $400$ psia

Choke size: $481/64$ inch

Flowline ID: $2$ In$.$

Gas production rate: $15000$ Mscf$/$d

Gas specific gravity: $0\mathrm{.}701$ for air

Gas specific heat ratio: $1\mathrm{.}25$

Upstream temperature: $120$ oF

Choke discharge coefficient: $1\mathrm{.}05$

Assuming gas viscosity is $0\mathrm{.}02$ cp and gas z$-$factor is $0\mathrm{.}9.$