The CAS Registry Mumber 4203-40-1 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 4,2,0 and 3 respectively; the second part has 2 digits, 4 and 0 respectively. Calculate Digit Verification of CAS Registry Number 4203-40: (6*4)+(5*2)+(4*0)+(3*3)+(2*4)+(1*0)=51 51 % 10 = 1 So 4203-40-1 is a valid CAS Registry Number.