Once you have got the name of the animal and the name of the client, you need to do the following:
i) Find the animal that the user selected in the animals hash; 
ii) Remove the animal from the animals hash; 
ii) Add the animal to the clients pets hash; 

    animal = shelter.animals[adopted]
    shelter.animals.delete(adopted)
    shelter.clients[adopter].num_pets[animal.name] = animal