The CAS Registry Mumber 81691-09-0 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 8,1,6,9 and 1 respectively; the second part has 2 digits, 0 and 9 respectively. Calculate Digit Verification of CAS Registry Number 81691-09: (7*8)+(6*1)+(5*6)+(4*9)+(3*1)+(2*0)+(1*9)=140 140 % 10 = 0 So 81691-09-0 is a valid CAS Registry Number.