Tc - Tf = 20
Tc = 20 + Tf
***************************
Tc/5=Tf-32/9
20 + Tf / 5 = Tf - 32/9
180 + 9Tf = 5Tf - 160
340 = -4Tf
Tf = 340/-4
*********************
Tf = -85°F.
*********************
Substituindo o valor e testando o exercicio.
Tc/5 = Tf-32/9
Tc/5 = -85-32/9
Tc/5 = -117/9
Tc/5 = -13
Tc= -13*5
***************
Tc= -65°C
***************