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How to write this math problem?
Solution: (1) If you build X medium-sized libraries, then build (30-x) small libraries.

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.