reserve X for set;
reserve a,b,c,k,m,n for Nat;
reserve i,j for Integer;
reserve r for Real;
reserve p,p1,p2 for Prime;

theorem Th16:
  n divides 50 implies n = 1 or n = 2 or n = 5 or n = 10 or n = 25 or n = 50
  proof
    assume n divides 50;
    then n divides 2*25;
    then consider a,b such that
A1: a divides 2 & b divides 25 and
A2: n = a*b by Lem1;
    (a = 1 or a = 2) & (b = 1 or b = 5 or b = 25) by A1,Th9,Th13;
    hence thesis by A2;
  end;
