How old is our baby bunny?

We found an abandoned baby bunny ( the rest of the littter is dead) and we are trying to save it but we cannot figure out how old it is. It still has part of the umbilical cord and it has teeth, it is about 3 1/2 inches long. The vet said that it will probably die but we are trying hard to save it. Any info on how to help it or if you think you may know how old it is is greatly appreciated. Thanks!!