Explanation

(a)
Let V litre of 10 N HCl be mixed with (1 - V) litre of 4 N HCl to give (V+1-V) = 1L of 7 N HCl.
$N_{1}V_{1 +}\ N_{2}V_{2} = N\ V$
10V + 4 (1-V)= $7\ \times \ 1\ \$
10V + 4 - 4V =7
6V = 7- 4
V= $\frac{3}{6}$ = 0.50L
Volume of 10 N HCl = 0.50L
Volume of 4N HCl = 1- 0.50=0.50 L