According to the meaning of the question: 3 points
Solution: 7 ≤ x ≤ 9.4 points.
∵ x is an integer ∴ x = 7, 8, 9, ∴ There are three schemes that meet the conditions. .................... scores 5 points.
(2) Building X Type A biogas digesters with a total cost of Y million yuan, then:
Y = 2x+3 (20-x) =-x+60 ... 6 o'clock.
∵- 1 & lt； 0, ∴y decreases with the increase of x,
When x=9, the value of y is the smallest, and y= 5 1 (ten thousand yuan) ............................................. 7 points.
At this time, the plan is: to build 9 Type A biogas digesters and 1 1 B biogas digesters, with 8 points in ........................................
Solution ②: As can be seen from (1), * * * has three solutions, and their costs are as follows:
Scheme 1: Build 7 Type A biogas digesters and 13 Type B biogas digesters.
The total cost is: 7×2+ 13×3 = 53 (ten thousand yuan) ...................................................... 6 points.
Scheme 2: Build 8 Type A biogas digesters and 12 Type B biogas digesters.
The total cost is: 8×2+ 12×3 = 52 (ten thousand yuan) .........................................................................................................................................
Scheme 3: build 9 A-type biogas digesters, 1 1 B-type biogas digesters.
The total cost is 9× 2+11× 3 = 51(ten thousand yuan).
∴ Scheme 3 is the most economical. .......................................................... scores 8 points.