x/40+3x/240=6

6x+3x= 1440

9x= 1440

x= 160

3x=3× 160=480

Steel is needed to make part a.

160÷40=4(m？ )

Steel is needed to make part B.

480÷240=2(m？ )

It will take x days to lay this pipeline.

x/ 12+x/24= 1

2x+x=24

3x=24

x=8

The two engineering teams will need eight days to build the pipeline from both ends at the same time.

3. Let the purchase price of small schoolbags be X yuan, and the direct purchase price of large schoolbags be x+ 10 yuan.

30%x=20%(x+ 10)

0.3x=0.2(x+ 10)

0.3x=0.2x+2

0.3x-0.2x=2

0. 1x=2

x=20

x+ 10=20+ 10=30

The price of small schoolbags is 20 yuan, and the price of big schoolbags is 30 yuan directly.