The CAS Registry Mumber 2309-03-7 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 2,3,0 and 9 respectively; the second part has 2 digits, 0 and 3 respectively. Calculate Digit Verification of CAS Registry Number 2309-03: (6*2)+(5*3)+(4*0)+(3*9)+(2*0)+(1*3)=57 57 % 10 = 7 So 2309-03-7 is a valid CAS Registry Number.