①、x+y = 500；

②、50x+80y = 28000；

X=400 and y= 100 are obtained by two equations ① and ②.

That is, A bought 400 yuan and B bought 100 yuan.

Question 2: set a: x b: y.

Meet the conditions: 1, x+y = 500;

2、50x+80y≤3400；

3. Let the survival number be m, then M=90%x+95%y, and this m should be the maximum.

The fourth equation 4, m = 475-50% x, can be obtained from 1 and 3;

5, x ≥ 200 can be obtained from 1 and 2;

From 4 and 5, it can be concluded that if m is the maximum, when x=200, m will be the maximum.

X=200, y = 300.

So to buy 200 saplings, 300 saplings should be the best choice.

I hope I can help you. I wonder if the solution is right. If it is different from your understanding, we can discuss it with each other again.