7. Two identical resistors with resistance $R$ each are connected in series and connected to a source of constant voltage $U$. An ideal voltmeter is connected in parallel to one of the resistors. Its readings were $U_{v}=15 B$. After that, the voltmeter was replaced with an ideal ammeter. The ammeter readings were $-I_{A}=20 \mathrm{~A}$. Determine the value of $R .(10$ points)
Answer: 1.5 Ohms

Solution. Voltage of the source: $U=U_{v}+U_{v}=30 V$ (4 points). The resistance of an ideal ammeter: $r_{A}=0$ Ohms (3 points). Therefore, the resistance of the resistor: $R=\frac{U}{I_{A}}=\frac{30}{20}=1.5$ Ohms (3 points).