The CAS Registry Mumber 18550-71-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,8,5,5 and 0 respectively; the second part has 2 digits, 7 and 1 respectively. Calculate Digit Verification of CAS Registry Number 18550-71: (7*1)+(6*8)+(5*5)+(4*5)+(3*0)+(2*7)+(1*1)=115 115 % 10 = 5 So 18550-71-5 is a valid CAS Registry Number.