How would I go about calculating the I^{-} concentration in a solution made up of 20 mL 0.20 M KI, 10 mL 0.0050 M Na_{2}S_{2}O_{3}, and 20 mL 0.10 M K_{2}S_{2}O_{8}?

When I first calculated I divided the mL of KI by the molar mass, so 20/166.0028=.1204 mol. Then I found the liters of solute by adding the mL of the other compounds to get 30 mL /1000=0.03L. Finally I divided .1204/0.03=4.013. However, I feel like this number is too large and I'm unsure where to go from here.