The CAS Registry Mumber 23115-91-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,3,1,1 and 5 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 23115-91: (7*2)+(6*3)+(5*1)+(4*1)+(3*5)+(2*9)+(1*1)=75 75 % 10 = 5 So 23115-91-5 is a valid CAS Registry Number.