[OrganicH2O]

If a solution is given as a percent, with no actual units given, what are the default units? I understand that percent could mean several different things, such as as Mass/Mass, Volume/Volume, or Mass/Volume. If Mass/Volume is used, the units seem arbitrary.

I see percent given on many different real world items, and the units are rarely provided.

[Corribus]

The percent is a dimensionless quantity.

[OrganicH2O]

I understand that it's typically a dimensionless quantity, but if the units are mass/volume, is that really dimensionless?

[Borek]

if the units are mass/volume, is that really dimensionless?

m/v% are faulty by design. This is just a way of describing the solution preparation procedure, not a real percentage.

If you want to be nitpicky you can assume their units to be g/mL% (whatever that means).

[Babcock_Hall]

This is a difficult question to answer, although there is a decent discussion in a very old quantitative analysis textbook by Pierce, Haenish, and Sawyer.  Context counts for quite a bit, because various sub-disciplines have conventions that are probably not known by those in other sub-disciplines.

IMO the author should always specify what is being measured and whether an initial or a final volume is meant.  If the author does not, then the experimenter must make an educated guess.  The substances were two liquids, I would assume volume to final volume of solution.  If the substances were a solid and a liquid, I would assume mass to final volume of solution.  You might want to ask yourself whether other assumptions would lead to small or large differences in strength of a solution.