其它
There were $20$ ducks, pigs, and sheep in total in Sam\textquotesingle s farm. After Sam bought some new sheep, the number of sheep has doubled. There are $27$ ducks, pigs, and sheep in total. Originally, how many ducks and pigs were there? 
Overseas Competition->Knowledge Point->Counting Modules
$27-20=7$  $7+7=14$  $20-14=6$ 