The CAS Registry Mumber 66121-97-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 6,6,1,2 and 1 respectively; the second part has 2 digits, 9 and 7 respectively. Calculate Digit Verification of CAS Registry Number 66121-97: (7*6)+(6*6)+(5*1)+(4*2)+(3*1)+(2*9)+(1*7)=119 119 % 10 = 9 So 66121-97-9 is a valid CAS Registry Number.