if you have only these data for every project, i think this table is good.
StudProject (StudNum, ProjNum, HoursWork,
DateStartWorkOnProj)
but if you want to store more information about project and work time, this table must be extended:
StudProject (StudNum, ProjNum)
projectWork (StudNum, ProjNum, workTime, startDateTime, endDateTime)
in the projectWork table, each record show a work day of student and difference of start, ned saved in workTime. sum(workTime) for each student in a project shows a total work of him.