A solution is $0.1\mathrm{M}$ in $\mathrm{HCl}$ and $0.1\mathrm{M}$ in $\mathrm{CH}{}_3\mathrm{COOH}$ ($K_a=1.8\times 10^{-5}$). Its pH is approximately

The strong acid suppresses the ionisation of acetic acid (common ion), so $[\mathrm{H}{}^{+}]\approx 0.1\mathrm{M}$ and $\mathrm{pH}\approx 1.00$. The value $2.87$ is the pH of acetic acid alone, which ignores the $\mathrm{HCl}$.