The CAS Registry Mumber 321172-72-9 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 3,2,1,1,7 and 2 respectively; the second part has 2 digits, 7 and 2 respectively. Calculate Digit Verification of CAS Registry Number 321172-72: (8*3)+(7*2)+(6*1)+(5*1)+(4*7)+(3*2)+(2*7)+(1*2)=99 99 % 10 = 9 So 321172-72-9 is a valid CAS Registry Number.