The CAS Registry Mumber 1640-73-9 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,6,4 and 0 respectively; the second part has 2 digits, 7 and 3 respectively. Calculate Digit Verification of CAS Registry Number 1640-73: (6*1)+(5*6)+(4*4)+(3*0)+(2*7)+(1*3)=69 69 % 10 = 9 So 1640-73-9 is a valid CAS Registry Number.