The CAS Registry Mumber 17655-66-2 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,7,6,5 and 5 respectively; the second part has 2 digits, 6 and 6 respectively. Calculate Digit Verification of CAS Registry Number 17655-66: (7*1)+(6*7)+(5*6)+(4*5)+(3*5)+(2*6)+(1*6)=132 132 % 10 = 2 So 17655-66-2 is a valid CAS Registry Number.