when we have Ksp CaSO4 = 9.1x10^-6 so by substitution in Ksp formula:
Ksp CaSO4 = [Ca+2][SO4]
1.9x10^-6 = [Ca+2][1.5x10^-2]
∴ [Ca+2] M in the solution =1.27x10^-4
when percent remaining = [Ca in solution]/[C original value] * 100
= [(1.27x10^-4*(50ml+50ml)]/[1.28x10^-2*50ml]*100
≈ 2%