In the given figure, switches S₁ and S₂ are in open condition. The resistance across ab when the switches S₁ and S₂ are closed is …… $\mathrm{\Omega }$.

When switches S₁ and S₂ are closed, then the given figure will be represented as follows

$\therefore$ The resistance across ab will be given as

$\begin{array}{l}{\mathrm{R}}_{\mathrm{ab}}=\frac{12\times 6}{12+6}+\frac{4\times 4}{4+4}+\frac{6\times 12}{6+12}\\ \Rightarrow {\mathrm{R}}_{\mathrm{ab}}=\frac{72}{18}+2+\frac{72}{18}\\ \Rightarrow {\mathrm{R}}_{\mathrm{ab}}=4+2+4=10\mathrm{\Omega }\end{array}$