In the <B>original</B> can of grape juice there was 500mg acsorbic acid along with sugar and water. Both the sugar and the acid have been dissolved and therefore do not affect volume.
*I just realized that I haven't taken into account the resulting change in volume from solid water to liquid water. If you think it's important, than I suggest that you find it.*
As the mass of ascorbic acid is given, we can use it to find the original concentration as you gave. (1.41 g/L)
The cans of water that are added are the same volume as the original can, therefore the final volume of the solution will be 355ml + 3(355mL) or 355ml * 4 which equals 1420ml or 1.42 L.
No extra solutes are added in the the latter solution, only the solvent water. The final concentration will simply be .5g/1.42L = .352 g/L
To finish the problem, find the percentage concentration of the final solution relative to the frozen can. 0.352g/L / 1.41g/L = 31% percent