The CAS Registry Mumber 1621-57-4 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,6,2 and 1 respectively; the second part has 2 digits, 5 and 7 respectively. Calculate Digit Verification of CAS Registry Number 1621-57: (6*1)+(5*6)+(4*2)+(3*1)+(2*5)+(1*7)=64 64 % 10 = 4 So 1621-57-4 is a valid CAS Registry Number.