...to project activities can be done in various ways. Think of an activity that can be done by a team of 6 people (long duration, low cost) or by a team of 3 persons and an expensive machine (short duration, high renting cost of the machine).
In the academic literature, this challenging problem is called the multi-mode resource constrained project scheduling problem, as follows:
At the Operations Research and Scheduling group, a lot of research has been done to optimize this scheduling process using Operations Research techniques. Click on the picture below to find the most recent results. Be careful: only for academics, or for project managers with an interest in what happens behind the ProTrack screens.