The CAS Registry Mumber 51956-21-9 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 5,1,9,5 and 6 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 51956-21: (7*5)+(6*1)+(5*9)+(4*5)+(3*6)+(2*2)+(1*1)=129 129 % 10 = 9 So 51956-21-9 is a valid CAS Registry Number.