Della decides to buy some stationaries for her students. The price of two schoolbags is equal to the price of six notebooks, and the price of six pencilcases is equal to nine notebooks. Given that one schoolbag can be exchanged for four pens, how many pens can be exchanged for one pencilcase? 
Overseas Competition->Knowledge Point->Word Problem Modules->Unitary Method Problems->Applying Multiplication and Division
$$1$$ schoolbag$$=3$$ notebooks$$=4$$ pens,  and $$6$$ pencilcases$$=9$$ notebooks,  Then, $$2$$ pencilcases$$=3$$ notebooks$$=4$$ pens.  So, $$1$$ pencilcase$$=2$$ pens. 