In order to realize the goal of building Shenyang into a forest city, the Greening Office plans to purchase and plant 400 saplings for a residential district in the greening work this spring.

( 1) y=400-2x

(2) According to the meaning of the question, 0.1x+0.4x = 0.2 (400-2x) ≥ 90.

x≥0

y≥0

The solution is 400≤x≤200.

Suppose the total cost of purchasing saplings is w = 3x+2x+3y = 5x+3 (400-2x) =-x+1200.

∵W decreases with the increase of x.

When x=200, the minimum value of ∴ W = 1000 yuan.

That is, when buying 200 poplars and 200 lilacs, and not buying willow, the total cost of buying saplings can be the lowest.

I hope I can help you.