The CAS Registry Mumber 204254-81-9 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 2,0,4,2,5 and 4 respectively; the second part has 2 digits, 8 and 1 respectively. Calculate Digit Verification of CAS Registry Number 204254-81: (8*2)+(7*0)+(6*4)+(5*2)+(4*5)+(3*4)+(2*8)+(1*1)=99 99 % 10 = 9 So 204254-81-9 is a valid CAS Registry Number.