Judging from the meaning of the question.
The solution of this inequality group is 18≤x≤20.
Because x can only be an integer, the value of ∴x is 18,19,20.
When x= 18, 30-x =12;
When x= 19, 30-x =11;
When x=20, 30-x= 10.
Therefore, there are three schemes: scheme 1, set 18 middle corner and 12 small corner;
Scheme 2: Set 19 middle book corner and 1 1 small book corner;
Scheme 3: Set 20 medium-sized corners and 10 small corners.
(2) Because the cost of setting medium-sized book corners is greater than that of setting small ones, the less the number of medium-sized book corners, the lower the cost, so the first scheme has the lowest cost, with the lowest cost of 860×18+570×12 = 22320.