The CAS Registry Mumber 42021-12-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 4,2,0,2 and 1 respectively; the second part has 2 digits, 1 and 2 respectively. Calculate Digit Verification of CAS Registry Number 42021-12: (7*4)+(6*2)+(5*0)+(4*2)+(3*1)+(2*1)+(1*2)=55 55 % 10 = 5 So 42021-12-5 is a valid CAS Registry Number.