Pat paints twice as fast as Lee. If it took Lee $6$ hours to paint a room, how long would it have taken if both had painted the room together? 
Overseas Competition->Knowledge Point->Word Problem Modules->Work Word Problems->Collaborative Work Word Problems
Lee paints the room once in $6$ hours. Pat paints the room twice in $6$ hours. Together, they paint the room $3$ times in $6$ hours. So, it takes them $2$ hours to paint it once together. 