The CAS Registry Mumber 6681-55-6 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,6,8 and 1 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 6681-55: (6*6)+(5*6)+(4*8)+(3*1)+(2*5)+(1*5)=116 116 % 10 = 6 So 6681-55-6 is a valid CAS Registry Number.