The CAS Registry Mumber 663-77-4 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 6,6 and 3 respectively; the second part has 2 digits, 7 and 7 respectively. Calculate Digit Verification of CAS Registry Number 663-77: (5*6)+(4*6)+(3*3)+(2*7)+(1*7)=84 84 % 10 = 4 So 663-77-4 is a valid CAS Registry Number.