The CAS Registry Mumber 84026-55-1 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 8,4,0,2 and 6 respectively; the second part has 2 digits, 5 and 5 respectively. Calculate Digit Verification of CAS Registry Number 84026-55: (7*8)+(6*4)+(5*0)+(4*2)+(3*6)+(2*5)+(1*5)=121 121 % 10 = 1 So 84026-55-1 is a valid CAS Registry Number.