The CAS Registry Mumber 76553-81-6 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,6,5,5 and 3 respectively; the second part has 2 digits, 8 and 1 respectively. Calculate Digit Verification of CAS Registry Number 76553-81: (7*7)+(6*6)+(5*5)+(4*5)+(3*3)+(2*8)+(1*1)=156 156 % 10 = 6 So 76553-81-6 is a valid CAS Registry Number.