The CAS Registry Mumber 999-77-9 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 9,9 and 9 respectively; the second part has 2 digits, 7 and 7 respectively. Calculate Digit Verification of CAS Registry Number 999-77: (5*9)+(4*9)+(3*9)+(2*7)+(1*7)=129 129 % 10 = 9 So 999-77-9 is a valid CAS Registry Number.