1. There is 200 grams of salt water with 8% concentration, and it takes (280) grams of salt water with 20% concentration to become 15% salt water.

Need to add (x) grams of 20% saline.

(200 * 8%+X * 20%)/(200+X)= 15%

16+0.2X = 0. 15(200+X)= 30+0. 15X

X= 14/0.05=280

2. Party A and Party B can complete a project within 12 days. If A did it for 2 days and B did it for 3 days, then they only finished 20% of all the work. If Party A and Party B do this project separately, it will take Party A 20 days and Party B 30 days.

Party A needs (x) days and Party B needs (y) days. ..

1/X+ 1/Y = 1/ 12-( 1)

2/X+3/Y =20%= 1/5 - (2)

(2)-(1) * 2:1/y =1/30-y = 30 (1)

X=20

3. For a project, Party A cooperates for 6 days to complete 5/6, and Party A does it alone. It takes the same time for Party A to complete 1/3 and Party B to complete 1/2, only 48 days for Party A and 12 days for Party B..

Party A and Party B sit for (x) days and (y) days respectively.

6/X+6/Y=5/6

X/3 = Y/2 = = = = = = 1/(3Y)- 1/(2X)= 0 = = = =-6/X+4/Y = 0

Add two types:

4. The distance between A and B is 960m, and they both start from A and B at the same time. If they meet within 6 minutes, it will take (1 1) minutes for B to catch up with A, just like walking in the opposite direction for 80 minutes.

It takes (x) minutes to get from a to b, and (y) minutes for b to get from a to b.

960( 1/X+ 1/Y)* 6 = 960 = = = 1/X+ 1/Y = 1/6

960 (1/x-1/y) 80 = 960 = =1/x-1/y =1/80 (sum difference problem)

1/X=8 1/80

These topics are difficult in junior high school. You should learn to formulate and solve binary linear equations, and have some skills to solve equations. You can practice several times independently.