3.8=0.05× 12+b+c

3.9=0.05×3^2+3b+c

b=-0. 15

c=3.9

∴ y = 0.05x2-0.15x+3.9 (1≤ x ≤1,where x is an integer);

(2) Let the profit per kilogram of goods transported be W yuan, which is deduced from the meaning of the question:

w=z-y=0.55x+6.45-(0.05x^2-0. 15x+3.9)，

=-0.05x^2+0.7x+2.55，

=-0.05(x-7)^2+5，

When x=7, the maximum value of w =5,

∴ in the seventh month, the profit per shipment 1 kg of goods is the largest, with the largest 5 yuan;

(3) When x= 1 1, y = 0.05×12-0.15×1+3.9 = 8.3.

∴ 165438+ 10/kg of goods transported in October is 1 2.5-8.3 = 4.2.

From the meaning of the title:12.5 (1+a%) (1-0.28a%)-8.3 (1+a%) = 4.2,

Let a%=m, then the original equation is deformed as: 3.5m 2-0.7m = 0,

Solution: m 1=0 (truncated), m2=0.2,

A: The value of A is 20.

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