Let I be the current for full scale deflection (50 divisions). The current for 20 divisions is I_g = (20/50)I = 0.4I. This is the current through the galvanometer. The current through the shunt is I_s = I - I_g = 0.6I. Since they are in parallel, I_g * G = I_s * S => 0.4I * G = 0.6I * 12. G = (0.6 * 12) / 0.4 = 18 Ω.