I haven't read through all the solutions yet, but I will.  But what I
learned most about this quiz is how not to propose the problem.

Had I requested that the microwave function take a string such as
"MM:SS" and return a similar string, and worded the quiz description
with that in mind, I think it would have eliminated much of the
initial confusion.

I figured that people would want to focus on the core problem and less
on string parsing and formatting, so I requested that the function
accept and return integers. But that caused more problems to save on
parsing a time string, which in retrospect is not that difficult (and
actually might have been a bit interesting).