The CAS Registry Mumber 109205-80-3 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,0,9,2,0 and 5 respectively; the second part has 2 digits, 8 and 0 respectively. Calculate Digit Verification of CAS Registry Number 109205-80: (8*1)+(7*0)+(6*9)+(5*2)+(4*0)+(3*5)+(2*8)+(1*0)=103 103 % 10 = 3 So 109205-80-3 is a valid CAS Registry Number.