What is the concentration of a solution prepared by adding 50.0 mL of a 3.0 M HCl solution to a 250 mL volumetric flask and filling to the mark with water?

To solve this we use the equation,

M1V1 = M2V2

where M1 is the concentration of the stock solution, V1 is the volume of the stock solution, M2 is the concentration of the new solution and V2 is its volume.

3.0 M (50 mL) = M2 (250 mL)

M2 = 0.6 M