The CAS Registry Mumber 21312-55-0 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,1,3,1 and 2 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 21312-55: (7*2)+(6*1)+(5*3)+(4*1)+(3*2)+(2*5)+(1*5)=60 60 % 10 = 0 So 21312-55-0 is a valid CAS Registry Number.