Solution: Let the willow seedling be X, the buttonwood seedling be Y and the pine seedling be Z..

x+y+z=900

x=3y

y=2z

X = 600, Y = 200 and Z = 100 are obtained.

A: There are 0/00 pine trees, 200 buttonwood trees and 600 willow trees.

The second question:

Solution: Assume that each group installs X telephones every day and Y telephones are newly installed every day.

3*60x=600+60y

5*20x=600+20y

X= 10，y=20。

Answer: Apply for 20 new phones every day, and each team will install 10 phones every day.

The third question:

Solution: Suppose you buy X copies for the first time and Y copies for the second time.

x+y=200

10x+ 10x * 0. 1+ 10 * . 085y = 1875

Solution: x=70, y= 130.

A: I bought 70 copies for the first time and 130 copies for the second time.