The CAS Registry Mumber 21000-12-4 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,1,0,0 and 0 respectively; the second part has 2 digits, 1 and 2 respectively. Calculate Digit Verification of CAS Registry Number 21000-12: (7*2)+(6*1)+(5*0)+(4*0)+(3*0)+(2*1)+(1*2)=24 24 % 10 = 4 So 21000-12-4 is a valid CAS Registry Number.