The CAS Registry Mumber 4402-91-9 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 4,4,0 and 2 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 4402-91: (6*4)+(5*4)+(4*0)+(3*2)+(2*9)+(1*1)=69 69 % 10 = 9 So 4402-91-9 is a valid CAS Registry Number.