The equivalent resistance between A and B as shown in figure is:

The equivalent circuit of the given configuration can be drawn as follows:

With reference to the above diagram, the equivalent resistance of the given configuration can be calculated as follows:
$\begin{cases}\dfrac{1}{R} & = & \dfrac{1}{20Ω} + \dfrac{1}{20Ω} + \dfrac{1}{10Ω} \\ & = & \dfrac{1}{5Ω} \\ & ⇒ & R = 5Ω\end{cases}$