Equal volumes of following solutions are mixed. In which case the pH of resulting solution will be average value of pH of two solutions?

Aqueous $\mathrm{HCl}$ of $\mathrm{pH}= 2$, aqueous $\mathrm{HCl}$ of $\mathrm{pH}= 4$

Aqueous $\mathrm{NaOH}$ of$\mathrm{pH} = 12$, aqueous $\mathrm{NaOH}$ of $\mathrm{pH}= 10$

Aqueous ${\mathrm{CH}}_3\mathrm{COOH}$ of $\mathrm{pH}= 5$, aqueous ${\mathrm{NH}}_3$ of $\mathrm{pH} = 9. [{\mathrm{K}}_{\mathrm{a}} ({\mathrm{CH}}_3\mathrm{COOH}) = \mathrm{Kb} ({\mathrm{NH}}_3\left)\right]$

Aqueous HCl of $\mathrm{pH}= 2$, aqueous $\mathrm{NaOH}$ of $\mathrm{pH}= 12$