The CAS Registry Mumber 107165-55-9 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,0,7,1,6 and 5 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 107165-55: (8*1)+(7*0)+(6*7)+(5*1)+(4*6)+(3*5)+(2*5)+(1*5)=109 109 % 10 = 9 So 107165-55-9 is a valid CAS Registry Number.