其它
Peter can draw a cartoon in $30$ minutes. How many hours does it take to paint four cartoons?~(adapted from 2011 Math kangaroo Problems, Level 1-2, Question \#4) 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem->Time Calculation
There are $60$ minutes in an hour.  It will take $120$ minutes to draw $4$ cartoons.~  $120$ minutes=$2$ hours. 