The CAS Registry Mumber 72731-13-6 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,2,7,3 and 1 respectively; the second part has 2 digits, 1 and 3 respectively. Calculate Digit Verification of CAS Registry Number 72731-13: (7*7)+(6*2)+(5*7)+(4*3)+(3*1)+(2*1)+(1*3)=116 116 % 10 = 6 So 72731-13-6 is a valid CAS Registry Number.