The CAS Registry Mumber 2109-54-8 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 2,1,0 and 9 respectively; the second part has 2 digits, 5 and 4 respectively. Calculate Digit Verification of CAS Registry Number 2109-54: (6*2)+(5*1)+(4*0)+(3*9)+(2*5)+(1*4)=58 58 % 10 = 8 So 2109-54-8 is a valid CAS Registry Number.