80x+30(30-x)≤ 1900 according to the meaning of the question.
50x+60(30-x)≤ 1620
The solution is 18≤x≤20.
X is a positive integer.
∴x= 18, 19,20
* * * There are three schemes: 1. Construction of 18 medium-sized library and 12 small library.
2. Build 19 medium-sized library and 1 1 small library.
3. Build 20 medium-sized libraries and 10 small libraries.
(2)∵860* 18+570* 12=22320
860* 19+570* 1 1=226 10
860*20+570* 10=22900
∴ Option 1 has the lowest cost of 22,320 yuan.