The CAS Registry Mumber 1741-21-5 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,7,4 and 1 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 1741-21: (6*1)+(5*7)+(4*4)+(3*1)+(2*2)+(1*1)=65 65 % 10 = 5 So 1741-21-5 is a valid CAS Registry Number.