The CAS Registry Mumber 727689-55-6 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 7,2,7,6,8 and 9 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 727689-55: (8*7)+(7*2)+(6*7)+(5*6)+(4*8)+(3*9)+(2*5)+(1*5)=216 216 % 10 = 6 So 727689-55-6 is a valid CAS Registry Number.