$$M_1V_1 = M_2V_2$$
Where:
M1 = Concentration of HBr solution (unknown)
V1 = Volume of HBr solution (12.0 mL)
M2 = Concentration of KOH solution (0.25 M)
V2 = Volume of KOH solution (15.0 mL)
Substituting the values into the formula, we get:
$$M_1 \times 12.0 \texttt{ mL} = 0.25 \texttt{ M} \times 15.0 \texttt{ mL}$$
Solving for M1, we get:
$$M_1 = \frac{0.25 \texttt{ M} \times 15.0 \texttt{ mL}}{12.0 \texttt{ mL}}$$
$$M_1 = 0.3125 \texttt{ M}$$
Therefore, the concentration of the HBr solution is 0.3125 M.
