The CAS Registry Mumber 109541-42-6 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,0,9,5,4 and 1 respectively; the second part has 2 digits, 4 and 2 respectively. Calculate Digit Verification of CAS Registry Number 109541-42: (8*1)+(7*0)+(6*9)+(5*5)+(4*4)+(3*1)+(2*4)+(1*2)=116 116 % 10 = 6 So 109541-42-6 is a valid CAS Registry Number.