Is the question perhaps worded a bit poorly?

It would seem that the AgNO3 cannot be in excess to solve, but is the exact amount that was required (probably a titration reaction). Since you can calculate the amount of chloride ions this consumes, you can calculate the amount of HCl used. The HCl was the stoichiometric amount, so this gives you the amount of cations reacted. Use the ratio of the mol. m of Zn and Mg to get the mix of cations consumed.