Transfer projects and merge similar projects to form (3) and (4).

Then (3) and (4) add up to eliminate one yuan.

3x - y = 8 (3)

5y-3x= 20 (4)

(3)+(4):

4y = 28

y= 7

Substitute y= 7 into equation (3)

x= 5

The second and third questions:

Formula (1) removes the brackets, the term shifts, the unknown belongs to the left of the equation, and the constant gets formula (3) on the right.

(2) Two sides of the formula are multiplied by 24 to eliminate the score, and the formula (4) is obtained.

That is:12x-21y =15 (3) (3) times 3 to complete the pin.

12x + 8y = 48 (4)

(4)-(3)

29 y = 33