The solubility of calcium sulfate at 30 degrees C is .209 g/100mL solution. calculate its Ksp.

CaSO4 ==> Ca^+2 + 2SO4^-2

Ksp + (Ca^+2)(SO4^-2)
Convert solubility in g/100mL to moles/L, substitute into Ksp expression and sole for Ksp.