The CAS Registry Mumber 20806-05-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,0,8,0 and 6 respectively; the second part has 2 digits, 0 and 5 respectively. Calculate Digit Verification of CAS Registry Number 20806-05: (7*2)+(6*0)+(5*8)+(4*0)+(3*6)+(2*0)+(1*5)=77 77 % 10 = 7 So 20806-05-7 is a valid CAS Registry Number.