The CAS Registry Mumber 6666-11-1 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,6,6 and 6 respectively; the second part has 2 digits, 1 and 1 respectively. Calculate Digit Verification of CAS Registry Number 6666-11: (6*6)+(5*6)+(4*6)+(3*6)+(2*1)+(1*1)=111 111 % 10 = 1 So 6666-11-1 is a valid CAS Registry Number.