The CAS Registry Mumber 7418-21-5 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 7,4,1 and 8 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 7418-21: (6*7)+(5*4)+(4*1)+(3*8)+(2*2)+(1*1)=95 95 % 10 = 5 So 7418-21-5 is a valid CAS Registry Number.