(2)P =(x-40)( 1600-20x)=-20x? +2400 x-64000 = 144000- 136000 = 8000 .
When x=-2400/(-20*2)=60,
Y has the maximum value =-20*60? +2400*60-64000= 144000- 136000=8000。
The solution is x 1=50, x2 = 70.
∵ The opening of parabola P=-20(x-60)2+8000 is downward,
When 50≤x≤70, the profit of selling zongzi every day is not less than 6000 yuan.
And ∵x≤58, ∴ when x=60, the maximum value of p =8000 yuan,
That is, when the price of each box is set as 60 yuan, the maximum profit is 8000 yuan;
(3)6000/(58-40)= 1000/3
The smallest integer greater than 1000/3 is 334.
That is, the supermarket sells at least 334 boxes of zongzi every day.