The CAS Registry Mumber 3353-33-1 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,3,5 and 3 respectively; the second part has 2 digits, 3 and 3 respectively. Calculate Digit Verification of CAS Registry Number 3353-33: (6*3)+(5*3)+(4*5)+(3*3)+(2*3)+(1*3)=71 71 % 10 = 1 So 3353-33-1 is a valid CAS Registry Number.