The CAS Registry Mumber 4321-91-9 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 4,3,2 and 1 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 4321-91: (6*4)+(5*3)+(4*2)+(3*1)+(2*9)+(1*1)=69 69 % 10 = 9 So 4321-91-9 is a valid CAS Registry Number.