The CAS Registry Mumber 18815-97-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,8,8,1 and 5 respectively; the second part has 2 digits, 9 and 7 respectively. Calculate Digit Verification of CAS Registry Number 18815-97: (7*1)+(6*8)+(5*8)+(4*1)+(3*5)+(2*9)+(1*7)=139 139 % 10 = 9 So 18815-97-9 is a valid CAS Registry Number.