a+b+c=2.6 ①

9a+3b+c=3.8 ②

16a+4b+c=5 ③

From ①, C = 2.6-a-b 4.

When ② is replaced by ④, 8a+2b+2.6=3.8.

∴ 8a+2b= 1.2

∴ 4a+b=0.6 ⑤

Replacing ③ with ④ means 15a+3b+2.6=5.

∴ 5a+b=0.8 ⑥

6-⑤ a=0.2

∴ b=-0.2

∴ c=2.6

the second question

Tip: When driving in opposite directions, the time when two cars pass by each other = (total length of two cars)/(sum of speed of two cars)

When driving in the same direction, the interval between two cars = (total length of two cars)/(speed difference between express train and local train)

From this point of view, it can be assumed that the speed of passenger cars is v 1 and the speed of freight cars is v2.

Judging from the meaning of the question: (150+250)/(v1+v2) =10.

(150+250)/(v1-v2) =100 (one minute and forty seconds = one hundred seconds)

By solving the equations, we get v1= 22m/sv2 =18m/s.

Third question

Tip: Profit = price difference * sales volume

y =(x-8)[ 100- 10(x- 10)]

=-10xx+280x-1600 (xx stands for the square of x, so the first question is over).

=- 10(x- 14)(x- 14)+360

∴ When x= 14, the maximum profit is obtained in 360 yuan.