Question

How many pens and how many pencils did he have before the trade

Answer 1

The answer will be 12 pens and pencils because ..."he only traded the pens then u should divided the pencils in 2 then u would get 12"... 
HOPE THIS HELPED BECAUSE I TRIED :)

Answer 2

He would have had 12 pens and 12 pencils before the trade