The CAS Registry Mumber 816-65-9 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 8,1 and 6 respectively; the second part has 2 digits, 6 and 5 respectively. Calculate Digit Verification of CAS Registry Number 816-65: (5*8)+(4*1)+(3*6)+(2*6)+(1*5)=79 79 % 10 = 9 So 816-65-9 is a valid CAS Registry Number.