The CAS Registry Mumber 2658-80-2 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 2,6,5 and 8 respectively; the second part has 2 digits, 8 and 0 respectively. Calculate Digit Verification of CAS Registry Number 2658-80: (6*2)+(5*6)+(4*5)+(3*8)+(2*8)+(1*0)=102 102 % 10 = 2 So 2658-80-2 is a valid CAS Registry Number.