(1) Let Part A be x cubic meters and Part B be (6-X) cubic meters.

40X =(6 X)240÷3

120X = 1440-240X

360X= 1440

X=4

Therefore, 40×4= 160 (pieces) is the number of a pieces.

240×(6-4)=480 (pieces) is the number of part B.

Therefore, (240+ 160)÷4= 160 (set)

A: It takes 160 A pieces and 480 B pieces to make 160 sets of such instruments.

(2) Because the working efficiency of A is112, the working efficiency of B is 1/24.

And because the total project ÷ working efficiency = working time.

Therefore, the unit "1" is taken as the general project of this problem.

So there are two problems that can be solved by equations:

Solution: Let the required number of days be X days.

X/ 12+X/24= 1

1/8X= 1

X=8 A: It will take 8 days to complete. ,1÷ (112+1/24) = 8 (days)

A: It takes 8 days. This problem is too simple to be solved by equations. )

(3) Let the purchase price of a big schoolbag be X yuan and that of a small schoolbag be (X- 10) yuan.

0.2X=(X- 10)0.3

-0. 1X=-3 30- 10=20 (yuan)

X=30

Answer: The purchase price of big schoolbag is 30 yuan, and that of small schoolbag is 20 yuan.

(4) set the number of copies as x.

0. 1X =(X-20)×0.09+20×0. 12

0. 1X=0.09X- 1.8+2.4

0.0 1X=0.6

X=60 A: the number of copies should be 60, and the charge is equal.