I agree that Mixed Numbers are a bit problematic in their form, but they are important for many applications.

Mixed numbers are non-standard in WeBWorK because it's confusing to parse, as you point out. And I agree that 3 + 1/3 removes all ambiguity.

These two problems in my Basic Algebra course actually require mixed number answers, not just allow them. That was what made this more confusing for my students. There was no way for them to get the problem correct unless they figured out to use the Test mode (as I now told them to do).

Paul