One pound of air at 100℉ is mixed with another pound of air at 40℉. the mixing process is constant pressure with constant specific heat. calculate: a. the final mixture temperature, ℉; ( 70℉ ) b. the change of entropy for the lb of gas originally at 100℉, btu/lb-°r; ( -0.01321 btu/lb°r ) c. the change of entropy for the lb of gas originally at 40℉, btu/lb-°r; ( +0.01398 btu/lb-°r ) d. the net change of entropy for the final mixture, ( +0.00077 btu/lb-°r )