That's the greedy(and obvious) solution, which is optimal for the fractional version of the problem, not in the case of the 0/1 problem, mentioned in the articl

That's the greedy(and obvious) solution, which is optimal for the fractional version of the problem, not in the case of the 0/1 problem, mentioned in the article. If you want a really challenging problem, look into the multiple knapsack problem.