Answer is: Osmosis of water from a low concentration of salt to a high concentration.
Osmosis is the spontaneous movement of solvent molecules (in this example water) through a selectively permeable membrane into a region of higher solute concentration (15 percent salt solution), in the direction that tends to equalize the solute concentrations on the two sides.
The direction of osmotic pressure is always from the side with the lower concentration of solute to the side with the higher concentration.