Suppose all the animals in the cage are chickens: there should be $$2\times 42=84$$ legs. However, $$124-84=40$$ legs are missing because we counted $$40\div2=20$$ rabbits. Hence, there are $$20$$ rabbits and $$42-20=22$$ chickens. 
A total of $$42$$ chickens and rabbits are caged together. If there are $$124$$ legs in total, how many chickens and rabbits are in the cage, respectively? 
  - $20$; $12$ 
  - $22$; $20$ 
  - $20$; $24$ 
  - $19$; $21$ 