Thank you nancythesleuth! You're exactly right. I've stopped considering it cheating to re-Continue until the challenge is at least doable.
What's even worse than the unobtainable challenges--like selling 10 meals on a slow day to a character-type who's been buying, say, 5 on a heavy day--are the ruinous ones, such as doubling your stock of an ingredient that you already have far more of than you need.
The other "cheat" that's probably worth the trouble is researching half your character types (and writing down the results) after your first day, starting over, researching the other half, restarting over, and then actually playing through the area.