The CAS Registry Mumber 75991-86-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,5,9,9 and 1 respectively; the second part has 2 digits, 8 and 6 respectively. Calculate Digit Verification of CAS Registry Number 75991-86: (7*7)+(6*5)+(5*9)+(4*9)+(3*1)+(2*8)+(1*6)=185 185 % 10 = 5 So 75991-86-5 is a valid CAS Registry Number.