$20\mathrm{.}27$ Consider a stack that emits a nonreactive pollutant at the rate of $55$g$//$s$.$ The stack is $85$ m tall, and is located in a rural area, surrounded for many miles by open pasture land. The plume rise is $35$ m $.$ The sky is cloudy on a November afternoon, and the wind speed $($measured at $10$ m height$)$ is $6\mathrm{.}0$m$//$s$.($a$)$ Esti$-$mate the downwind, centerline, ground$-$level concentration at a downwind distance of $5000$ meters from this stack. Give your answer in mug$//$m$^(3).($b$)$ Also, estimate the ground$-$level concentra$-$tion at a cross$-$wind distance of $200$ m $,$ but still at the same downwind distance of $5000$ m $.$ Give your answer in mug$//$m$^(3).$