The CAS Registry Mumber 76663-23-5 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 7,6,6,6 and 3 respectively; the second part has 2 digits, 2 and 3 respectively. Calculate Digit Verification of CAS Registry Number 76663-23: (7*7)+(6*6)+(5*6)+(4*6)+(3*3)+(2*2)+(1*3)=155 155 % 10 = 5 So 76663-23-5 is a valid CAS Registry Number.