Oni has $$11$$ penpals. Last week she wrote to all of them.  She wrote a $$4$$-page letter to some of her penpals and a $$3$$-page letter to the rest. Altogether she wrote $$38$$ pages.  To how many penpals did Oni write a $$3$$-page letter? 
Overseas Competition->Knowledge Point->Word Problem Modules->Chicken-Rabbit Problems->Solving Chicken-Rabbit Problems by Using Hypothesis->Basic Type->Non-typical Types