Question 1170322
Means, John needs 24 hours alone and Justine needs 28 hours alone.  
(You say  "within 24 hour" so maybe you need an inequality instead of equations.)
(I'm ignoring the "within" part.)


John        {{{ 1/24}}}


Justine      {{{1/28}}}


If both together, speed is {{{1/24+1/28}}}, units of PROBLEM SET per HOURS.
Work with it!


Actually if question  was only How long will it take Justine to answer, that was already given almost plainly.  She needs no more than 28 hours.