The CAS Registry Mumber 3687-99-8 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,6,8 and 7 respectively; the second part has 2 digits, 9 and 9 respectively. Calculate Digit Verification of CAS Registry Number 3687-99: (6*3)+(5*6)+(4*8)+(3*7)+(2*9)+(1*9)=128 128 % 10 = 8 So 3687-99-8 is a valid CAS Registry Number.