reserve a,b,c,k,m,n for Nat;
reserve p for Prime;

theorem
  20 satisfies_Sierpinski_problem_76b
  proof
    let x be Nat;
    assume 10*20 < x < 10*(20+1);
    then 200 < x < 209+1;
    then 200+1 <= x <= 209 by NAT_1:13;
    then x = 201 or ... or x = 209;
    hence thesis by XPRIMES0:201,202,203,204,205,206,207,208,209;
  end;
