The CAS Registry Mumber 71635-31-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,1,6,3 and 5 respectively; the second part has 2 digits, 3 and 1 respectively. Calculate Digit Verification of CAS Registry Number 71635-31: (7*7)+(6*1)+(5*6)+(4*3)+(3*5)+(2*3)+(1*1)=119 119 % 10 = 9 So 71635-31-9 is a valid CAS Registry Number.