Maharashtra State PSC Model Paper 2

Concept:
When the resistances are connected in series, the equivalent resistance is given by
${\mathit{R}}_{\mathit{e}\mathit{q}}={\mathit{R}}_1+{\mathit{R}}_2+...+{\mathit{R}}_{\mathit{n}}$
The equivalent resistance is greater than the largest resistance in the series circuit
When the resistances are connected in parallel, the equivalent resistance is given by
$\frac{1}{{\mathit{R}}_{\mathit{e}\mathit{q}}}=\frac{1}{{\mathit{R}}_1}+\frac{1}{{\mathit{R}}_2}+...+\frac{1}{{\mathit{R}}_{\mathit{n}}}$
The equivalent resistance is lesser than the smallest resistance in the series circuit
Calculation:
${\mathit{R}}_1=9\mathrm{ohm},{\mathit{R}}_2=4\mathrm{ohm},{\mathit{R}}_3=12$
$\frac{1}{{\mathit{R}}_{\mathit{e}\mathit{q}}}=\frac{1}{9}+\frac{1}{4}+\frac{1}{12}$
$\Rightarrow {\mathit{R}}_{\mathit{e}\mathit{q}}=2.2\mathit{\Omega }$