The CAS Registry Mumber 66521-35-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 6,6,5,2 and 1 respectively; the second part has 2 digits, 3 and 5 respectively. Calculate Digit Verification of CAS Registry Number 66521-35: (7*6)+(6*6)+(5*5)+(4*2)+(3*1)+(2*3)+(1*5)=125 125 % 10 = 5 So 66521-35-5 is a valid CAS Registry Number.