The CAS Registry Mumber 1641-50-5 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 1 respectively; the second part has 2 digits, 5 and 0 respectively. Calculate Digit Verification of CAS Registry Number 1641-50: (6*1)+(5*6)+(4*4)+(3*1)+(2*5)+(1*0)=65 65 % 10 = 5 So 1641-50-5 is a valid CAS Registry Number.