The CAS Registry Mumber 81391-20-0 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 8,1,3,9 and 1 respectively; the second part has 2 digits, 2 and 0 respectively. Calculate Digit Verification of CAS Registry Number 81391-20: (7*8)+(6*1)+(5*3)+(4*9)+(3*1)+(2*2)+(1*0)=120 120 % 10 = 0 So 81391-20-0 is a valid CAS Registry Number.