The CAS Registry Mumber 6709-40-6 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 6,7,0 and 9 respectively; the second part has 2 digits, 4 and 0 respectively. Calculate Digit Verification of CAS Registry Number 6709-40: (6*6)+(5*7)+(4*0)+(3*9)+(2*4)+(1*0)=106 106 % 10 = 6 So 6709-40-6 is a valid CAS Registry Number.