The CAS Registry Mumber 51-33-2 includes 5 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 2 digits, 5 and 1 respectively; the second part has 2 digits, 3 and 3 respectively. Calculate Digit Verification of CAS Registry Number 51-33: (4*5)+(3*1)+(2*3)+(1*3)=32 32 % 10 = 2 So 51-33-2 is a valid CAS Registry Number.