The CAS Registry Mumber 17909-28-3 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,7,9,0 and 9 respectively; the second part has 2 digits, 2 and 8 respectively. Calculate Digit Verification of CAS Registry Number 17909-28: (7*1)+(6*7)+(5*9)+(4*0)+(3*9)+(2*2)+(1*8)=133 133 % 10 = 3 So 17909-28-3 is a valid CAS Registry Number.