The CAS Registry Mumber 2656-03-3 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 6 respectively; the second part has 2 digits, 0 and 3 respectively. Calculate Digit Verification of CAS Registry Number 2656-03: (6*2)+(5*6)+(4*5)+(3*6)+(2*0)+(1*3)=83 83 % 10 = 3 So 2656-03-3 is a valid CAS Registry Number.