The CAS Registry Mumber 3940-66-7 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,9,4 and 0 respectively; the second part has 2 digits, 6 and 6 respectively. Calculate Digit Verification of CAS Registry Number 3940-66: (6*3)+(5*9)+(4*4)+(3*0)+(2*6)+(1*6)=97 97 % 10 = 7 So 3940-66-7 is a valid CAS Registry Number.