The final molarity of a solution made by mixing $$50\,mL$$  of $$0.5\,M\,HCl,150\,mL$$     of $$0.25\,M\,HCl$$    and water to make the volume $$250\,mL$$   is

Solution :
$$\eqalign{ & {M_1}{V_1} + {M_2}{V_2} = MV \cr & 0.5 \times 50 + 0.25 \times 150 = M \times 250 \cr & M = \frac{{25 + 37.5}}{{250}} \cr & \,\,\,\,\,\,\,\, = 0.25\,M \cr} $$