The CAS Registry Mumber 3684-81-9 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,6,8 and 4 respectively; the second part has 2 digits, 8 and 1 respectively. Calculate Digit Verification of CAS Registry Number 3684-81: (6*3)+(5*6)+(4*8)+(3*4)+(2*8)+(1*1)=109 109 % 10 = 9 So 3684-81-9 is a valid CAS Registry Number.