The CAS Registry Mumber 101980-55-6 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,0,1,9,8 and 0 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 101980-55: (8*1)+(7*0)+(6*1)+(5*9)+(4*8)+(3*0)+(2*5)+(1*5)=106 106 % 10 = 6 So 101980-55-6 is a valid CAS Registry Number.