The formula for mitigation on armor is: Mitigation = (Armor*0.0004)/(1+(Armor*0.0004))*100
Solving for 75% ...
75 = AX / (1+AX) * 100
0.75 = AX / (1+AX)
0.75(1+AX) = AX
0.75 + 0.75 AX = AX
0.75 = AX - 0.75 AX
0.75 = 0.25AX
0.75/0.25X = A = 0.75/(0.0004*(.25)) = 7500