Solve for y...?

Question

Solve for y:

x = 7y

x + y + z = 80

2x - y + z = 105

(A) - 7

(B) 15

(C) - 175 / 13

(D) 35

(E) none of these

Answer 1

Replace x by 7y in the second and third equations to get

8y + z = 80

13y + z = 105

Subtract the equations to obtain -5y = -25

So, y = 5

Answer 2

Use the Properties of Equality to isolate the y on one side of the equation - your choice.