The equivalent resistance $R_{\text{eq}}$ is $8\ \Omega$.

The equivalent resistance $R_{\text{eq}}$ is equal to exactly half of the harmonic mean of the two individual resistances.

The equivalent resistance $R_{\text{eq}}$ is $25\ \Omega$.

The equivalent resistance is given by the formula $R_{\text{eq}} = \frac{R_1 R_2}{R_1 + R_2}$.