Can someone please explain the concept of indistinguishable items within a combination problem, such as the one below:

Ben needs to form committee of 3 from a group of 8 engineers. If 2 of the engineers are too inexperienced to serve in the same committee, how many different committees can Ben form?
- 20
- 30
- 50
- 56
- 336

I think the answer is 50, but I have no idea how that was derived. Thanks!

PS. This question can be found in the Princeton Review - Manual for the GMAT ver. 8.2 p. 346

Thanks!