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

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. 
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.