The CAS Registry Mumber 180616-21-1 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,8,0,6,1 and 6 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 180616-21: (8*1)+(7*8)+(6*0)+(5*6)+(4*1)+(3*6)+(2*2)+(1*1)=121 121 % 10 = 1 So 180616-21-1 is a valid CAS Registry Number.