The CAS Registry Mumber 28831-66-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,8,8,3 and 1 respectively; the second part has 2 digits, 6 and 6 respectively. Calculate Digit Verification of CAS Registry Number 28831-66: (7*2)+(6*8)+(5*8)+(4*3)+(3*1)+(2*6)+(1*6)=135 135 % 10 = 5 So 28831-66-5 is a valid CAS Registry Number.