The CAS Registry Mumber 401-91-2 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 4,0 and 1 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 401-91: (5*4)+(4*0)+(3*1)+(2*9)+(1*1)=42 42 % 10 = 2 So 401-91-2 is a valid CAS Registry Number.