The CAS Registry Mumber 56553-71-0 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 5,6,5,5 and 3 respectively; the second part has 2 digits, 7 and 1 respectively. Calculate Digit Verification of CAS Registry Number 56553-71: (7*5)+(6*6)+(5*5)+(4*5)+(3*3)+(2*7)+(1*1)=140 140 % 10 = 0 So 56553-71-0 is a valid CAS Registry Number.