The CAS Registry Mumber 128665-98-5 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,2,8,6,6 and 5 respectively; the second part has 2 digits, 9 and 8 respectively. Calculate Digit Verification of CAS Registry Number 128665-98: (8*1)+(7*2)+(6*8)+(5*6)+(4*6)+(3*5)+(2*9)+(1*8)=165 165 % 10 = 5 So 128665-98-5 is a valid CAS Registry Number.