The CAS Registry Mumber 91021-23-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 9,1,0,2 and 1 respectively; the second part has 2 digits, 2 and 3 respectively. Calculate Digit Verification of CAS Registry Number 91021-23: (7*9)+(6*1)+(5*0)+(4*2)+(3*1)+(2*2)+(1*3)=87 87 % 10 = 7 So 91021-23-7 is a valid CAS Registry Number.