(2)y=500-0.8x≤280 → x≥275

X and y are integers.

When x=275, y=280, that is, A buys 275 cases and B buys 280 cases.

When x=276, y=279, that is, A buys 276 cases and B buys 279 cases.

When x=277, y=278, that is, A buys 277 cases and B buys 278 cases.

When x=278, y=277, that is, A buys 278 cases and B buys 277 cases.

When x=279, y=276, that is, A buys 279 cases and B buys 276 cases.

When x=280, y=276, that is, A buys 280 cases and B buys 276 cases.

(Abandoning the scheme of x=279 and y=276, there are five purchase schemes in the supermarket. )

(3) That is, Party A sells a box of 19.2 yuan and Party B sells a box of 25 yuan.

A earns 3.2 yuan per case, and B earns 5 yuan per case.

∴ The profits of the above scheme are 2280 yuan, 2278.2 yuan, 2276.4 yuan, 2274.6 yuan and 2276 yuan in turn, so when Party A buys 275 cases and Party B buys 280 cases, the maximum profit is 2280 yuan. In fact, this problem, because B's profit per box is greater than A, so we should choose B's biggest plan.

④50A+ 100 b = 400

→A=6，B= 1

A=4，B=2

A=2，B=3