Explanation

(a) $\frac{1}{R_{1}} = \frac{1}{10} + \frac{1}{2.5} = \frac{5}{10} = \frac{1}{2} \Rightarrow R_{1} = 2\Omega$
Now $2\Omega$ and $10\ \Omega\ $are in series
$R_{2} = 10 + 2 = 12\Omega$
$R_{2}\ $and $12\Omega\ $are in parallel
$\frac{1}{R_{3}} = \frac{1}{12} + \frac{1}{12} \Rightarrow R_{3} = 6\Omega$
Now $R_{3}$and $6\Omega$ are in series
$R_{4} = 10 + 6 = 16\Omega$
Now, $R_{4}$and $16\ \Omega$ are in parallel
$\therefore\ \frac{1}{R} = \frac{1}{16} + \frac{1}{16}$
$\Rightarrow \ R = 3\Omega$