Billy has twice as many basketballs as soccer balls.  Milly has four times as many soccer balls as basketballs.  They have a total of 18 balls.  How many of them are basketballs? 
Overseas Competition->Knowledge Point->Word Problem Modules->Equation Word Problems->Indefinite Equation Word Problems
Let Billy have $$b$$ soccer balls and $$2b$$ basketballs. Let Milly have $$m$$ basketballs and $$4m$$ soccer balls.  Therefore, as they have $$17$$ balls in total, $$3b + 5m= 18$$. The only positive integer solution of this equation is $$b= 1$$, $$m= 3$$. So the number of basketballs is $$2b + m = 2 \times 1 + 3 = 5$$. 