Arwen originally had 12 pens.
If Arwen obtains 3 new pens from Adrienne, Arwen now has 15 pens. If Adrienne still has 6 more after giving Arwen 3 pens, that means she has 21 pens. And without giving the 3 pens, she would have 24, which is half of what Arwen originally had.
