I think of two numbers, x and y

x-y is half of x+y

Write x in terms of y

You said that (x - y) is half of (x + y). I can work with that.

(x - y) = 1/2 (x + y)

x - y = 1/2 x + 1/2 y

Multiply each side by 2:

2x - 2y = x + y

Subtract 'x' from each side:

x - 2y = y

Add 2y to each side:

x = 3y

They can be ANY two numbers, as long as 'x' is triple whatever 'y' is.

Let's try a couple:

x = 18, y = 6

(x - y) = 12

(x + y) = 24

12 = 1/2 of 24x

yay!

x = 30, y = 10

(x - y) = 20

(x + y) = 40

20 = 1/2 of 40

yay!

x = 3π, y = π

(x - y) = 2π

(x + y) = 4π

2π = 1/2 of 4π

yay!

X-y = x+y all over 2

Add the y onto the other side making x = x+y all over 2 + y

Then you times everything by 2

2x = x+2y

Subtract x from both sides giving you

X=2y