The CAS Registry Mumber 688-76-6 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 6,8 and 8 respectively; the second part has 2 digits, 7 and 6 respectively. Calculate Digit Verification of CAS Registry Number 688-76: (5*6)+(4*8)+(3*8)+(2*7)+(1*6)=106 106 % 10 = 6 So 688-76-6 is a valid CAS Registry Number.