Calculate initial concentrations of both Ca2+ and SO42-. When CaSO4 precipitates both concentrations go down by the same amount (because of the stoichiometry). They go down till their product hits Ksp. Try to express it as an equation - with one unknown (amount of precipitate). Then it should be a breeze.
I'm still having lots of trouble with this question. I got [SO4] = 0.0074 M and [Ca2+] = 0.0062
Then I did ([SO4][Ca2+]) - x = 9.1 x 10^-6
so x = 3.68 x 10^-5 M = amount both concentrations go down by correct?
So I substracted this from the initial concentration, then divided by total, then x 100... my answers make no sense