The CAS Registry Mumber 26641-42-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,6,6,4 and 1 respectively; the second part has 2 digits, 4 and 2 respectively. Calculate Digit Verification of CAS Registry Number 26641-42: (7*2)+(6*6)+(5*6)+(4*4)+(3*1)+(2*4)+(1*2)=109 109 % 10 = 9 So 26641-42-9 is a valid CAS Registry Number.