The CAS Registry Mumber 16661-24-8 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,6,6,6 and 1 respectively; the second part has 2 digits, 2 and 4 respectively. Calculate Digit Verification of CAS Registry Number 16661-24: (7*1)+(6*6)+(5*6)+(4*6)+(3*1)+(2*2)+(1*4)=108 108 % 10 = 8 So 16661-24-8 is a valid CAS Registry Number.