Answer: The concentration of a mixture of two or more liquids is expressed as a percentage by volume.
Reason:
Concentration of solution in terms of percentage can be expressed in two ways
1) percentage by mass
2) percentage by volume
Choice of method is dependent on physical state of matter. In case of liquid, it is more easy to measure volume (using burette, pipette, standard measuring flask, etc.) as compared to mass.
Hence, for liquid solutions, concentration is expressed in terms of percentage by volume.