of ten boxes, five contain pencils, four contain pens, and two contain both pencils and pens. How many boxes are empty??

Answer 1

Your question isn't clear.

Evidently, the two containing both pencils and pens are included in the first part of the question. Therefore, only one box may be empty.

Answer 2

Or is it possible the two that contain pens and pencils can kind of cross over. 3 boxes Pens only, 2 boxes pencils only, 2 boxes pens and pencils combined. So that would leave 3 boxes empty ? That would also work out to 5 boxes with pens, 4 boxes with pencils and 2 with both and remain under 10.