The CAS Registry Mumber 18831-81-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,8,8,3 and 1 respectively; the second part has 2 digits, 8 and 1 respectively. Calculate Digit Verification of CAS Registry Number 18831-81: (7*1)+(6*8)+(5*8)+(4*3)+(3*1)+(2*8)+(1*1)=127 127 % 10 = 7 So 18831-81-7 is a valid CAS Registry Number.