The CAS Registry Mumber 684-26-4 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 6,8 and 4 respectively; the second part has 2 digits, 2 and 6 respectively. Calculate Digit Verification of CAS Registry Number 684-26: (5*6)+(4*8)+(3*4)+(2*2)+(1*6)=84 84 % 10 = 4 So 684-26-4 is a valid CAS Registry Number.