The CAS Registry Mumber 31991-64-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 3,1,9,9 and 1 respectively; the second part has 2 digits, 6 and 4 respectively. Calculate Digit Verification of CAS Registry Number 31991-64: (7*3)+(6*1)+(5*9)+(4*9)+(3*1)+(2*6)+(1*4)=127 127 % 10 = 7 So 31991-64-7 is a valid CAS Registry Number.