Answer
Two $10\Omega$ resistors in series.
Work Step by Step
First find the equivalent resistance of five $100\Omega$ resistors in parallel.
When resistors are wired in parallel, we use equation 19–4 to find the equivalent resistance.
$$\frac{1}{R_{eq}}=\frac{1}{100\Omega}+\frac{1}{100\Omega}+\frac{1}{100\Omega}+\frac{1}{100\Omega}+\frac{1}{100\Omega}$$
$$R_{eq}=20\Omega$$
Next consider the $10\Omega$ resistors in series. When resistors are wired in series, we use equation 19–3 to find the equivalent resistance. The resistances add.
By dividing, $(20\Omega/10\Omega)$, we see that two $10\Omega$ resistors in series add up to give us the required $20\Omega$ resistance.