The equivalent resistance between $\mathrm{A}$ and $\mathrm{B}$ 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:
$\begin{array}{rcl}\frac{1}{R}& =& \frac{1}{20 \mathrm{\Omega }}+\frac{1}{20 \mathrm{\Omega }}+\frac{1}{10 \mathrm{\Omega }}\\ & =& \frac{1}{5 \mathrm{\Omega }}\\ & \Rightarrow & R= 5\mathrm{\Omega }\end{array}$