The CAS Registry Mumber 6672-17-9 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,6,7 and 2 respectively; the second part has 2 digits, 1 and 7 respectively. Calculate Digit Verification of CAS Registry Number 6672-17: (6*6)+(5*6)+(4*7)+(3*2)+(2*1)+(1*7)=109 109 % 10 = 9 So 6672-17-9 is a valid CAS Registry Number.