Here's the problem,
Calculate the concentrations of each ion in a solution that results from mixing 50 mL of a 0.2 M NaClO3 solution with 25 mL of a 0.20 M Na2SO4 solution.
The units always tell you what to do. You want molarities, which is moles divided by liters, so get moles. Get Liters. Divide. Viola!