## Ducks and dogs

*Difficulty level:*

In one place there are 21 animals, between ducks and dogs. Being 54 the total feet of these animals, calculate the difference between the number of ducks and the number of dogs.

*Difficulty level:*

In one place there are 21 animals, between ducks and dogs. Being 54 the total feet of these animals, calculate the difference between the number of ducks and the number of dogs.