The CAS Registry Mumber 21-72-7 includes 5 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 2 digits, 2 and 1 respectively; the second part has 2 digits, 7 and 2 respectively. Calculate Digit Verification of CAS Registry Number 21-72: (4*2)+(3*1)+(2*7)+(1*2)=27 27 % 10 = 7 So 21-72-7 is a valid CAS Registry Number.