The CAS Registry Mumber 23905-57-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,3,9,0 and 5 respectively; the second part has 2 digits, 5 and 7 respectively. Calculate Digit Verification of CAS Registry Number 23905-57: (7*2)+(6*3)+(5*9)+(4*0)+(3*5)+(2*5)+(1*7)=109 109 % 10 = 9 So 23905-57-9 is a valid CAS Registry Number.