The CAS Registry Mumber 21420-59-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,1,4,2 and 0 respectively; the second part has 2 digits, 5 and 9 respectively. Calculate Digit Verification of CAS Registry Number 21420-59: (7*2)+(6*1)+(5*4)+(4*2)+(3*0)+(2*5)+(1*9)=67 67 % 10 = 7 So 21420-59-7 is a valid CAS Registry Number.