The CAS Registry Mumber 66271-26-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 6,6,2,7 and 1 respectively; the second part has 2 digits, 2 and 6 respectively. Calculate Digit Verification of CAS Registry Number 66271-26: (7*6)+(6*6)+(5*2)+(4*7)+(3*1)+(2*2)+(1*6)=129 129 % 10 = 9 So 66271-26-9 is a valid CAS Registry Number.