The CAS Registry Mumber 17166-16-4 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,7,1,6 and 6 respectively; the second part has 2 digits, 1 and 6 respectively. Calculate Digit Verification of CAS Registry Number 17166-16: (7*1)+(6*7)+(5*1)+(4*6)+(3*6)+(2*1)+(1*6)=104 104 % 10 = 4 So 17166-16-4 is a valid CAS Registry Number.