The CAS Registry Mumber 6689-16-3 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 9 respectively; the second part has 2 digits, 1 and 6 respectively. Calculate Digit Verification of CAS Registry Number 6689-16: (6*6)+(5*6)+(4*8)+(3*9)+(2*1)+(1*6)=133 133 % 10 = 3 So 6689-16-3 is a valid CAS Registry Number.