The answers to questions 8, 6, 7 and 9 in the second volume of the seventh grade mathematics book published by People's Education Press.

Solution: let a be x weeks/minute and b be y weeks/minute.

Countable equation:

2x+2y= 1 ①

6x-6y= 1 ②

① Multiply by 3: 6x+6y = 3 ③

②+③ De: 12x = 4x = 1/3

Substituting x = 3 into ① gives 2 * 1/3+2y = 1.

2/3+2y= 1

y= 1/6

∴ The solution of this set of equations is:

x= 1/3

y= 1/6

A: A runs a third lap every minute, and B runs a sixth lap every minute.

Solution: X block of type A steel plate is needed, and Y block of type B steel plate is needed.

Then 2x+y= 15 ①

X+2y= 18 ②

The solution is x=4.

y=7

A: We need 4 A-type steel plates and 7 B-type steel plates.

Solution: Let the original length of the spring be xcm and the forward coefficient be KN/m. According to the meaning of the question, we can get:

k( 16.4-x)=2 ①

k( 17.9-x)=5 ②

According to formula ①, k=2/( 16.4-x) ③.

Substitute ③ into ② to get:

3x=46.2

Then x= 15.4.

Answer: The spring should be15.4cm..