The CAS Registry Mumber 6713-91-3 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,7,1 and 3 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 6713-91: (6*6)+(5*7)+(4*1)+(3*3)+(2*9)+(1*1)=103 103 % 10 = 3 So 6713-91-3 is a valid CAS Registry Number.