Atypical hydrogen peroxide solution from a drug store is labeled as 3.0% hydrogen peroxide. what is the other 97% of this composed of?

Water

Explanation:

Chemically speaking, a solution has two components: solute and solvent.

The solute is that substance (usually solid, but it can also be liquid or gas) that dissolves in a liquid substance and its proportion in the resulting solution is usually less than the solvent.

The solvent is the substance used to dissolve the solute, and generally its proportion in the resulting solution is greater. Being the most common used  solvent water.

This means that in a solution there is usually more solvent than solute.

So, in the case of the hydrogen peroxide solution, since the proportion is to , the hydrogen peroxide is the solute and water is the solvent.

they both have a y in them