The CAS Registry Mumber 128720-09-2 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,2,8,7,2 and 0 respectively; the second part has 2 digits, 0 and 9 respectively. Calculate Digit Verification of CAS Registry Number 128720-09: (8*1)+(7*2)+(6*8)+(5*7)+(4*2)+(3*0)+(2*0)+(1*9)=122 122 % 10 = 2 So 128720-09-2 is a valid CAS Registry Number.