The CAS Registry Mumber 1391913-21-5 includes 10 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 7 digits, 1,3,9,1,9,1 and 3 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 1391913-21: (9*1)+(8*3)+(7*9)+(6*1)+(5*9)+(4*1)+(3*3)+(2*2)+(1*1)=165 165 % 10 = 5 So 1391913-21-5 is a valid CAS Registry Number.