The CAS Registry Mumber 55590-42-6 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 5,5,5,9 and 0 respectively; the second part has 2 digits, 4 and 2 respectively. Calculate Digit Verification of CAS Registry Number 55590-42: (7*5)+(6*5)+(5*5)+(4*9)+(3*0)+(2*4)+(1*2)=136 136 % 10 = 6 So 55590-42-6 is a valid CAS Registry Number.