The CAS Registry Mumber 3347-21-5 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,3,4 and 7 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 3347-21: (6*3)+(5*3)+(4*4)+(3*7)+(2*2)+(1*1)=75 75 % 10 = 5 So 3347-21-5 is a valid CAS Registry Number.