@134318c2 
two of my math books (I think pre-algebra and algebra I) each had about a chapter's worth of exercise sets devoted to modest rearrangements of this sort. At the time I lived in a chaotic and abusive home, and so did not have any reasonable time or place to do homework, and was thus not fond of any kind of homework that seemed overly obvious or too time-consuming, but I could see the necessity of learning to easily do such modest rearrangements.