[tex]\tt Se~da:\\ \\
U=220V\\ \\
h=30min=1800s\\ \\
L=4kWh=14400000J\\ \\
R=?\Omega\\ \\ \\
Formule:\\ \\
L=P \times t\\ \\
L=\frac{U^{2}}{R}\times t\Rightarrow R=\frac{U^{2}\times t}{L}\\ \\ \\
Calcule:\\ \\
R=\frac{220 \times 1800}{14400000}=6,05\Omega
[/tex]