The CAS Registry Mumber 1680-28-0 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,6,8 and 0 respectively; the second part has 2 digits, 2 and 8 respectively. Calculate Digit Verification of CAS Registry Number 1680-28: (6*1)+(5*6)+(4*8)+(3*0)+(2*2)+(1*8)=80 80 % 10 = 0 So 1680-28-0 is a valid CAS Registry Number.