The CAS Registry Mumber 664371-61-3 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 6,6,4,3,7 and 1 respectively; the second part has 2 digits, 6 and 1 respectively. Calculate Digit Verification of CAS Registry Number 664371-61: (8*6)+(7*6)+(6*4)+(5*3)+(4*7)+(3*1)+(2*6)+(1*1)=173 173 % 10 = 3 So 664371-61-3 is a valid CAS Registry Number.