(a)
According to molarity equation
NaOH = HCl
${\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ M}_{1}V_{1\ } = \ M_{2}V_{2}$
$0.6\ \times V_{1\ }\ = \ 0.4\ \times \ 30\ \ \$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ V_{1\ }\ = \ \frac{0.4\ \times \ 30\ }{0.6\ \ } = 20{cm}^{3}$