The CAS Registry Mumber 135980-77-7 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, 7 and 7 respectively. Calculate Digit Verification of CAS Registry Number 135980-77: (8*1)+(7*3)+(6*5)+(5*9)+(4*8)+(3*0)+(2*7)+(1*7)=157 157 % 10 = 7 So 135980-77-7 is a valid CAS Registry Number.