Question 905849
Substitute the numbers for x and y. This will be a system of equations, which means that there will be two or more equations.

x= one of the numbers
y= one of the numbers

x+y= 80
x-y=1/2(x+y)

Substitute x+y for 80.
x-y=1/2(80)
x-y=40

Now that you've simplified the equation, here are the two equations:
x+y=80
x-y=40

You can subtract the two equations from each other:
  (x+y=80)
- (x-y=40)
----------
     2y=40
      y=20

Because y=20, x=60 since y+x=80.