The CAS Registry Mumber 135980-91-5 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,3,5,9,8 and 0 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 135980-91: (8*1)+(7*3)+(6*5)+(5*9)+(4*8)+(3*0)+(2*9)+(1*1)=155 155 % 10 = 5 So 135980-91-5 is a valid CAS Registry Number.