The CAS Registry Mumber 204638-28-8 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 2,0,4,6,3 and 8 respectively; the second part has 2 digits, 2 and 8 respectively. Calculate Digit Verification of CAS Registry Number 204638-28: (8*2)+(7*0)+(6*4)+(5*6)+(4*3)+(3*8)+(2*2)+(1*8)=118 118 % 10 = 8 So 204638-28-8 is a valid CAS Registry Number.