Suppose the number of chickens is $x$, so the number of rabbits is $(20-x)$.  $2x+(20-x)\times4=56$, $x=12$. 
A total of $$20$$ chickens and rabbits are caged together. If there are $$56$$ legs in total, how many chickens are there in the cage? 
Options:
  * $$20$$ 
  * $$16$$ 
  * $$15$$ 
  * $$12$$ 
  * $$10$$ 