其它
Vera leaves home at $$9.15$$ a.m. to walk to Derek's house, which takes her $$25$$ minutes. Carl leaves his house $$5$$ minutes after Vera but only takes $$6$$ minutes to get to Derek's house.  When Carl arrives, how long will he and Derek have to wait for Vera to arrive? 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem
Vera will arrive at $$9.40$$ a.m.  Carl will leave at $$9.20$$ a.m. and arrive at $$9.26$$ a.m.  So, Carl and Derek will have to wait for $$14$$ min. 