The CAS Registry Mumber 7657-53-6 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 7,6,5 and 7 respectively; the second part has 2 digits, 5 and 3 respectively. Calculate Digit Verification of CAS Registry Number 7657-53: (6*7)+(5*6)+(4*5)+(3*7)+(2*5)+(1*3)=126 126 % 10 = 6 So 7657-53-6 is a valid CAS Registry Number.