Explanation:
If we connect battery between \(A\) and \(B\), then different points will be at the same potentials.

\((1,2,3,4),(5,6,7,8),(9,10\), \(11,12,13,14,15,16\) ), and \((17,18,19,20)\)The resulting circuit can be drawn as follows:

\(R_{A B}=\dfrac{21}{40} r=\dfrac{21}{40} \times \dfrac{80}{7}=6 \Omega\)