The CAS Registry Mumber 866-15-9 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 8,6 and 6 respectively; the second part has 2 digits, 1 and 5 respectively. Calculate Digit Verification of CAS Registry Number 866-15: (5*8)+(4*6)+(3*6)+(2*1)+(1*5)=89 89 % 10 = 9 So 866-15-9 is a valid CAS Registry Number.