$20\; ml$ of $0.1\;M$ acetic acid is mixed with $50\;ml$ of Potassium acetate. $K_a$ of Acetic acid = $1.8 \times 10^{-5}$ at $27^{\circ}C$. The concentration of Potassium acetate if pH of the mixture is $4.8$:

Question

$$\begin {array} {1 1} (1)\;0.1\;M & \quad (2)\;0.04\;M \\ (3)\;0.4\;M & \quad (4)\;0.02\;M \end {array}$$