The CAS Registry Mumber 21188-82-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,1,1,8 and 8 respectively; the second part has 2 digits, 8 and 2 respectively. Calculate Digit Verification of CAS Registry Number 21188-82: (7*2)+(6*1)+(5*1)+(4*8)+(3*8)+(2*8)+(1*2)=99 99 % 10 = 9 So 21188-82-9 is a valid CAS Registry Number.