Anna went to travel by bike. She started at $8$ am. But she had to stop for $15$ minutes on the way and arrived at $12$ am. How long did Anna ride her bicycle? (adapted from 2011 Math kangaroo Problems, Level 3-4 , Question \#4) 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem->Time Calculation
$$12:00-8:00=4$$ h  $4$h- $15$min =$3$ h $45$min 