Answer:
From the original 20 boxes, since 3 have neither pens nor pencils, let's put those boxes aside. Now we have only 17 boxes.
Of these 17 boxes, some many have only pens, some may have only pencils, and some may have both pens and pencils.
We know that 13 of them have pencils and since every box that we now have has either pens or pencils or both, the 4 boxes that don't have pencils must have pens.
But, since 9 boxes have pens (subtracting 4 boxes from 9 boxes, giving us 5 boxes), 5 of these boxes must be boxes that also have pencils.
If we look inside all the boxes, we will find that 8 boxes have only pencils, 5 boxes have both pens and pencils, and 4 boxes have only pens. (And 3 that have neither pens nor pencils.)
