The CAS Registry Mumber 65999-55-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 6,5,9,9 and 9 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 65999-55: (7*6)+(6*5)+(5*9)+(4*9)+(3*9)+(2*5)+(1*5)=195 195 % 10 = 5 So 65999-55-5 is a valid CAS Registry Number.