Voltage across a circuit

For two resistors placed in parallel, the total resistance is $R_{pt} = \frac{R_{1}R_{2}}{R_{1}+R_{2}}$ (here $pt$ is simply the total resistance of the parallel part of the circuit). A superconductor by definition has zero resistance, so $R_{pt} = 0$ in the parallel part. Taking the parallel part as a "resistor" with zero resistance, the 15-ohm resistor connected in series is the only one contributing to the resistance of the circuit. Thus, $R_{total} = 15 \text{ ohms}$ . A 5-ampere current is passed through the circuit. Therefore, through $V = IR$ , substituting $I$ for 5 amperes and $R$ for 15 ohms, $V$ is then 75 volts.