If they can work together, but not on the same problems, jill should work for 3/2 hours to complete 6 chemistry problems, but stop doing the math problems after 9/7 hours, having complete 9 maths problems.

After 4/3 hours of working together, jack has completed 8 math problems and should stop doing math problems, but continue doing chemistry problems for a further 1/3 hours, completing 5 chemistry problems in total.

In this way, their combined efforts have (optimally) resulted in the completion of 11 chemistry problems and 17 math problems, as required.