I think that&#x27;s by far the fastest way to solve #4.  Seeing as how I didn&#x27;t have a pencil, I wasn&#x27;t inclined to solve the system algebraically.  I just ruled out C&amp;D for being too large, and figured it was B since C was only just barely too large, and A was clearly too small.  I think this is a legitimate way to solve the problem and a useful problem as well.  I&#x27;d be happy if I met more people who could estimate and guess at math solutions.