Ah, alright - didn't know that!
My confusion is how to work out the added concentration of the Zn2+ ions...What I know is that x/(x+y) = 0.35/0.46 (x = conc. of zinc, y = conc. of zinc ions), I know that y can be worked out and I...think that after finding x the final answer can be reached (before conversion) through (x * 500) / 10 (i.e. total sample amount divided by sample size) but I can't reach the answer that is given as the right one. I've messed around with multiplying/dividing the various values (0.05, 5.00, 200/500 or 2/5) and the value I should get for y is achieved through (0.05 * 2 * 5 / 0.4) but without knowing where the *2 comes from I can't really go with that. I've also tried to work out the concentration through using the molar mass of the zinc ions (5 / molar mass) equalling 250/3269 then (moles / 0.05) which gets 1.52 which still doesn't let me reach the answer...