The statement of the problem seems to be incomplete. It looks line the 0.15 is the molality of the solution.

Then 0.15 = x mol of S_8 / 100.0 g of solvent, so that x = 0.015 mol of S_8.

To find the mass of sulfur S_8, multiply the number of moles by its molar mass:

0.015 mol * (8*32.06) g/mol = 3.8 g.

Note: In order to find the answer in case when 0.15 represents the molarity of the solution or other concentration unit we should know the density of the solution.