X*0.9 + Y = 10 - 0.8
x+Y & gt； 10
If X=9
Then 9*0.9 = 8. 1.
Add change 0.8 = 8.9
So the price of milk is 1. 1 yuan.
No discount, the price is
9+ 1. 1 & gt； 10 meets the requirement of not buying enough milk.
If X=8
Then 8*0.9 = 7.2
Add change 0.8 = 8
So the price of milk is 2 yuan.
No discount, the price is
8+2 = 10 does not meet the requirement of not buying enough milk.
If X=7
Then 7*0.9 = 6.3.
Add 0.8 = 7. 1.
So the price of milk is 2.9 yuan.
No discount, the price is
7+2.9 & lt； 10 does not meet the requirement of not buying enough milk.
If X=6
Then 6*0.9 = 5.4.
Plus change 0.8 = 6.2.
So the price of milk is 3.8 yuan.
No discount, the price is
6+3.8 & lt； 10 does not meet the requirement of not buying enough milk.
If X=5
Then 5*0.9 = 4.5.
Plus 0.8 = 5.3.
So the price of milk is 4.7 yuan.
No discount, the price is
5+4.7 & lt； 10 does not meet the requirement of not buying enough milk.
……
So the answer is
Cookie 9 yuan, milk 1. 1 yuan.