A linear equation [x (1+50%)+(600-x) (1+40%)] × 90%-600 =174 is obtained from the problem.
Solution: x=200
600 x = 400
2.( 1) can be obtained from the question: y=400-(x-40)×80.
y=400-80x+3200
y=-80x+3600
(2) Substitute 380 barrels into (1) to get the relation: 380=-80x+3600.
-80x=-3220
x=40.25
Then: the total annual cost of this kind of purified water is: 40.25×380+780= 16075 yuan.
The total cost of drinks is: 50× 120=6000 yuan.
So it is more cost-effective to use drinks.
3.( 1) Solution: Let the painting area of each room be x square meters.
Then: [(4x-44)/3]-10 = (6x+14)/7.
Solution: (4x-74)/3=(6x+ 14)/7.
28x-5 18= 18x+42
10x=560
x=56
(2) According to the problem, the daily painting area of the first-class mechanic is: (56× 4-44) ÷ 3 = 60m2.
Secondary mechanic: 60- 10 = 50m2.
For every Y first-class technicians, there are 20-y second-class technicians.
Then: 60y+(20-y)50=20×56.
60y+ 1000-50y = 1 120
10y= 120
y= 12
20-y=8
Question 4: I really lack the conditions about traffic flow, or I can't do it.