The CAS Registry Mumber 6506-66-7 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,5,0 and 6 respectively; the second part has 2 digits, 6 and 6 respectively. Calculate Digit Verification of CAS Registry Number 6506-66: (6*6)+(5*5)+(4*0)+(3*6)+(2*6)+(1*6)=97 97 % 10 = 7 So 6506-66-7 is a valid CAS Registry Number.