Solution: (1) (Analysis: it can be increased by 1 yuan per kilogram per day, with the original market price of 30 yuan per kilogram).

After freezing for x days, the market unit price is (30+x) yuan.

(2) (Analysis: After X days, there are 10x grams of fruits, with blackening value of 10* 10x, and the remaining 1000- 10x are good fruit values (1000-.

If p is the sales volume, then

p = 10 * 10x+( 1000- 10x)(30+x)-400 x

= 100 x+30000+ 1000 x-300 x- 10x？ -400 times

=30000+400x- 10x？

(3) (Analysis: You can know the maximum profit by replacing the above formula with a vertex)

P=30000+400x- 10x？

=- 10(x？ -40x+400)+34000

=- 10(x-20)？ +34000

When x=20, p gets the maximum value: P=34000.