Judging from the meaning in the title: 0.5x+0.8(6000-x)=3600,

Solve this equation and get: x=4000,

∴6000-x=2000，

Answer: A buys 4000 fry, B buys 2000 fry;

(2) From the meaning of the question: 0.5x+0.8(6000-x)≤4200,

Solve this inequality, x≥2000,

That is, no less than 2000 fry should be purchased;

(3) Let the total cost of buying fry be y 。

Y=0.5x+0.8(6000-x)=-0.3x+4800，

From the meaning of the question, there are 90100x+95100 (6000-x) ≥ 93100x 6000.

Solution: x≤2400,

In y=-0.3x+4800,

∫-0.3 < 0, ∴y decreases with the increase of x,

When x=2400, y is minimum = 4080.

Answer: Buy 2400 first-class fry and 3600 second-class fry, with the lowest total cost.