The CAS Registry Mumber 13643-33-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,3,6,4 and 3 respectively; the second part has 2 digits, 3 and 3 respectively. Calculate Digit Verification of CAS Registry Number 13643-33: (7*1)+(6*3)+(5*6)+(4*4)+(3*3)+(2*3)+(1*3)=89 89 % 10 = 9 So 13643-33-9 is a valid CAS Registry Number.