The CAS Registry Mumber 20521-07-7 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 2,0,5,2 and 1 respectively; the second part has 2 digits, 0 and 7 respectively. Calculate Digit Verification of CAS Registry Number 20521-07: (7*2)+(6*0)+(5*5)+(4*2)+(3*1)+(2*0)+(1*7)=57 57 % 10 = 7 So 20521-07-7 is a valid CAS Registry Number.