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There are n solutions.

1 suppose x cars are modified for the first time, and the daily gasoline cost of each car after modification is y yuan.

List 2 equations:

xy/(( 100-x)80)=3/20

2xy/(( 100-2x)80)=2/5

The solution is x = 20 y = 48.

(80-48)/80=0.4

4000/32= 125

So * * * modified 40 cars, and the average daily fuel cost of each modified taxi decreased by 40% compared with that before modification.

If the company refits all taxis at one time, the cost can be recovered from the saved fuel cost after 125 days.

2 sets, changed x cars for the first time, saved y yuan, according to the meaning of the question, you can get

1、XY=( 100-X)*3/20*80 - A

2XY=( 100-2X)*2/5*80 - B

2XY =( 100-2X)* 32-XY = 1600-32X-C

c = A-( 100-X)* 12 = 1600-32X

Solution, X=20 vehicles, Y=48 yuan, reduced by (80-48)/80=40%.

2. After all the modifications,

100 * 4000 ÷100 * (80-48) =125 days

Option 3: Suppose X cars are modified for the first time, and the daily gasoline cost of each car after modification is Y yuan.