The CAS Registry Mumber 16561-27-6 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,6,5,6 and 1 respectively; the second part has 2 digits, 2 and 7 respectively. Calculate Digit Verification of CAS Registry Number 16561-27: (7*1)+(6*6)+(5*5)+(4*6)+(3*1)+(2*2)+(1*7)=106 106 % 10 = 6 So 16561-27-6 is a valid CAS Registry Number.