The CAS Registry Mumber 3909-27-1 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 3,9,0 and 9 respectively; the second part has 2 digits, 2 and 7 respectively. Calculate Digit Verification of CAS Registry Number 3909-27: (6*3)+(5*9)+(4*0)+(3*9)+(2*2)+(1*7)=101 101 % 10 = 1 So 3909-27-1 is a valid CAS Registry Number.