I think that's by far the fastest way to solve #4. Seeing as how I didn't have a pencil, I wasn't inclined to solve the system algebraically. I just ruled out

I think that's by far the fastest way to solve #4. Seeing as how I didn't have a pencil, I wasn't inclined to solve the system algebraically. I just ruled out C&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'd be happy if I met more people who could estimate and guess at math solutions.