The CAS Registry Mumber 1021987-10-9 includes 10 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 7 digits, 1,0,2,1,9,8 and 7 respectively; the second part has 2 digits, 1 and 0 respectively. Calculate Digit Verification of CAS Registry Number 1021987-10: (9*1)+(8*0)+(7*2)+(6*1)+(5*9)+(4*8)+(3*7)+(2*1)+(1*0)=129 129 % 10 = 9 So 1021987-10-9 is a valid CAS Registry Number.