(a+b)/2 = 18； (a+c)/2 = 17； (b+c)/2= 19
From these three formulas, we can know that A = 16, B = 20 and C = 18.
A: A planted 16 trees, B planted 20 trees and C planted 18 trees.
18. solution: suppose there are y people in group b, and according to the meaning of the question, (160y+140× 6) =152 (6+y), y=9.
A: there are 9 people in group B.
19. solution: let this field be yμ, and the meaning in the question is: (85y+1kloc-0/.5× 5) = 92.5 (5+y), y=6.
This land covers 6 hectares.
20. solution: we assume that the average is y yuan per kilogram. According to the meaning of the question, (2y+4×8)=7(4+y) gives y=0.8.
A: Average 0.8 yuan per kilogram.