A computer store sells new computers for $500 and refurbished computers for $200. In March, the store sold 20 computers for $6,400, meaning they sold ?
refurbished computers.

no of new computers sold : 8

no. of refurbished computer sold : 12

Step-by-step explanation:

Let the no. of new computers sold be x

let the no. of refurbished computers sold be y

Given

In March, the store sold 20 computers

x + y = 20

y = 20-x equation 2

selling price of new computer = $500

selling price of x new computer = 500*x = 500x

selling price of refurbished computer = $200

selling price of y refurbished computer = 200*y = 200y

Total selling price of x new computer and y refurbished computers = 500x+200y

given that

total prioce of computer is $6400

thus

500x+200y = 6400

using y = 20-x  from equation 2

500x+200(20-x) = 6400

=> 500x+ 4000 - 200x = 6400

=> 300x = 6400 - 4000 = 2400

=> x = 2400/300 = 8

Thus,

no of new computers sold = 8

no. of refurbished computer sold = 20 -8 = 12

2nd one in the first row is correct

x = -3

y = -18

step-by-step explanation:

to solve by substitution, substitute one variable and solve for the other. then substitute again to find the remaining variable.

x-1/3(3x-9) = -3 since y=3x-9 substittute y.

x-1/3(3x-9)=-3

x + x +3 = -3

2x+3=-3

2x+3-3= -3-3

2x=-6

x=-3

substitute again into y=3x-9

y=3(-3)-9

y=-9-9

y=-18