The CAS Registry Mumber 55556-56-4 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 5,5,5,5 and 6 respectively; the second part has 2 digits, 5 and 6 respectively. Calculate Digit Verification of CAS Registry Number 55556-56: (7*5)+(6*5)+(5*5)+(4*5)+(3*6)+(2*5)+(1*6)=144 144 % 10 = 4 So 55556-56-4 is a valid CAS Registry Number.