Change in mass number = 232 - 220 = 12. This requires 12/4 = 3 alpha particles. Ejecting 3 alpha particles decreases the atomic number by 3×2=6. Expected atomic number = 92-6 = 86. The actual atomic number is 89. To increase it from 86 to 89, 3 beta decays (which increase atomic number by 1 each) are required.