The CAS Registry Mumber 2178-18-9 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 2,1,7 and 8 respectively; the second part has 2 digits, 1 and 8 respectively. Calculate Digit Verification of CAS Registry Number 2178-18: (6*2)+(5*1)+(4*7)+(3*8)+(2*1)+(1*8)=79 79 % 10 = 9 So 2178-18-9 is a valid CAS Registry Number.