The CAS Registry Mumber 97998-91-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 9,7,9,9 and 8 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 97998-91: (7*9)+(6*7)+(5*9)+(4*9)+(3*8)+(2*9)+(1*1)=229 229 % 10 = 9 So 97998-91-9 is a valid CAS Registry Number.