30.0 mL of 0.10 M Ca(NO3)2 and 15.0 mL of 0.20 M Na3PO4 solutions are mixed. After the reaction is complete, which of these ions has the lowest concentration in the final solution?
A) Na+ B) NO3- C) Ca+2 D) PO4-3
Answer is C.. How do you solve it though?!