Why do solvent particles flow into the cell when the initial volume is below 50%?

Answer: Differences in osmotic concentrations

Explanation: With solvent particles flowing into the cell, it means the concentration outside of the cell is higher and with initial volume less than 50%, then that within the cell is lower. This results in an osmotic gradient, allowing particles in areas of higher concentration (outside the cell) to flow into the cell, an area of lower concentration.

Different in concentration

Explanation:

When the initial cell volume is below 50%, the concentration of solvent inside the cell is less than the concentration of solvent outside the cell. Hence the solvent particles outside the cell tend to move into the cell because of it's lower concentration.