The CAS Registry Mumber 1020056-91-0 includes 10 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 7 digits, 1,0,2,0,0,5 and 6 respectively; the second part has 2 digits, 9 and 1 respectively. Calculate Digit Verification of CAS Registry Number 1020056-91: (9*1)+(8*0)+(7*2)+(6*0)+(5*0)+(4*5)+(3*6)+(2*9)+(1*1)=80 80 % 10 = 0 So 1020056-91-0 is a valid CAS Registry Number.