Do u know the title of this book?

I can't remember the title of this book I read awhile ago. It was about this girl who bought a dog without her parents knowing so she had to hid it and get money to feed it. Then she had an old lady look after the dog while she wad at school then she ended up giving the dog to the lady. I think its part of the animal ark books if that helps. Please if u know the name of the book that would b great!!! Thx so much! :)