The CAS Registry Mumber 677-42-9 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 6,7 and 7 respectively; the second part has 2 digits, 4 and 2 respectively. Calculate Digit Verification of CAS Registry Number 677-42: (5*6)+(4*7)+(3*7)+(2*4)+(1*2)=89 89 % 10 = 9 So 677-42-9 is a valid CAS Registry Number.