The CAS Registry Mumber 33938-21-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 3,3,9,3 and 8 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 33938-21: (7*3)+(6*3)+(5*9)+(4*3)+(3*8)+(2*2)+(1*1)=125 125 % 10 = 5 So 33938-21-5 is a valid CAS Registry Number.