The CAS Registry Mumber 99166-39-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 9,9,1,6 and 6 respectively; the second part has 2 digits, 3 and 9 respectively. Calculate Digit Verification of CAS Registry Number 99166-39: (7*9)+(6*9)+(5*1)+(4*6)+(3*6)+(2*3)+(1*9)=179 179 % 10 = 9 So 99166-39-9 is a valid CAS Registry Number.