The CAS Registry Mumber 523-17-1 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 5,2 and 3 respectively; the second part has 2 digits, 1 and 7 respectively. Calculate Digit Verification of CAS Registry Number 523-17: (5*5)+(4*2)+(3*3)+(2*1)+(1*7)=51 51 % 10 = 1 So 523-17-1 is a valid CAS Registry Number.