Answer
Connect two 40-ohm resistors in parallel.
Work Step by Step
The equation is not given in this textbook, but for 2 resistors in parallel, the following relation holds.
$$\frac{1}{R_{parallel}} = \frac{1}{R_{1}} + \frac{1}{R_{2}}$$
Apply the equation to this problem.
$$\frac{1}{R_{parallel}} = \frac{1}{40 \Omega} + \frac{1}{40 \Omega } = \frac{1}{20 \Omega} $$
$$R_{parallel} = 20 \Omega$$
