Let's call one number x and one number y.
x/2=y
x+y=57
We can set these equations equal to each other to solve for the numbers by subtracting x from both sides of the second equation. Now the equations are:
y=-x+57 and x/2=y
Since both equations equal y, they are also equal to each other:
-x+57=x/2
First, multiply both sides by 2:
-2x+114=x
Add 2x to both sides.
114=3x
Divide both sides by 3.
38=x
One of the numbers is 38. Now, we can plug 38 in for x in either of the equations to solve for y:
(38)+y=57
Subtract 38 from both sides:
y=19
The two numbers are 38 and 19.
I hope this helps :)