You're right. 800 yuan is not used correctly. It should be an inequality of 20X+35Y≤800.

But if you think so, you should buy as much as possible if you want to get the maximum profit. Of course, the more money you buy, the more profit you make. It's just a general concept. So this problem can be described as: 20X+35Y≤800, y >: =7, and the maximum value 10X+ 13Y is required.