The CAS Registry Mumber 19909-10-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,9,9,0 and 9 respectively; the second part has 2 digits, 1 and 0 respectively. Calculate Digit Verification of CAS Registry Number 19909-10: (7*1)+(6*9)+(5*9)+(4*0)+(3*9)+(2*1)+(1*0)=135 135 % 10 = 5 So 19909-10-5 is a valid CAS Registry Number.