Carbon dioxide flows in a horizontal adiabatic, no-work system. Pressure and temperature at section 1 are \(7 \mathrm{~atm}\) and \(600 \mathrm{~K}\). At a downstream section, \(p_{2}=4 \mathrm{~atm}\)., \(T_{2}=550 \mathrm{~K}\), and the Mach number is \(M_{2}=0.90\).

(a) Compute the velocity at the upstream location.

(b) What is the entropy change?

(c) Determine the area ratio \(A_{2} / A_{1}\).