Answer:
33.3%
Step-by-step explanation:
You want the mass volume % of a solution of 5 grams of solute in 30 grams of solution that has a density of 2 g/mL.
The volume of the solution is its mass divided by its density.
V = m/ρ
V = (30 g)/(2 g/mL) = 15 mL
Then the ratio of mass of solute to volume of solution is ...
(5 g)/(15 mL) = 1/3 g/mL
Expressed as a percentage, that is ...
(1/3 g/mL) · 100% = 33.3% mass/volume
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Additional comment
It is usually easier to measure the volume of a liquid solution than to measure its mass. The quantity of interest in a solution is often the mass of the solute it contains, so a mass/volume ratio has a certain practicality.