R1=(1/5);
R2=(0.45/0.8);
R3=(diz/0.04);
R4=(1/23);
Solve[(1/(R1+R2+R3+R4))==0.3];
ΔT=40;
P1=0.3*ΔT;
Tstena=20-P1*R1

(4200/3600.)