The CAS Registry Mumber 135518-28-4 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,3,5,5,1 and 8 respectively; the second part has 2 digits, 2 and 8 respectively. Calculate Digit Verification of CAS Registry Number 135518-28: (8*1)+(7*3)+(6*5)+(5*5)+(4*1)+(3*8)+(2*2)+(1*8)=124 124 % 10 = 4 So 135518-28-4 is a valid CAS Registry Number.