The CAS Registry Mumber 115666-55-2 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,1,5,6,6 and 6 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 115666-55: (8*1)+(7*1)+(6*5)+(5*6)+(4*6)+(3*6)+(2*5)+(1*5)=132 132 % 10 = 2 So 115666-55-2 is a valid CAS Registry Number.