Not an easy problem. I wonder how my perfectionist kid, DD16, made it through Everyday Math. She did so without complaining, and even now she doesn't think it was a bad experience. DD9 is in her first year of public school and hates Everyday Math. Complains they don't spend enough time on any one topic and they throw random stuff in that they haven't learned yet (such as primes). Previously she was in a school that used a European math text (French). While the French do "spiral" in math, they spend more time on each topic and the explanations are much better than I have seen in American texts.
To do that problem properly, you need to know some geometry. I'd call it an 8th grade problem.
