Would sound travel faster through a gas that is 90% nitrogen and 10% helium or through a gas that is 90% helium and 10% gas

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.