A10-milliliter solution containing 2 grams of drug is a % solution.

Percentage by mass is the amount in mass of a component in a mixture per 100 unit of mass of the total mixture. Percentage by mass is the same as  %w/w. We can determine this by dividing the mass of the solute with the total mass of the mixture. From the problem statement, the mass of the component is given as 2 g and the mass of the total mixture is 10 milliliter. It would be difficult to express this in percent by mass since we do not have data in the density. We can only express this in percent by w/v which is grams per 100mL of solution.

%w/v = 2 g / 10mL x 100 = 20% w/v solution