This falls under things you wouldn't bother worrying about in real life.
Swap Two Integers without using a Temporary Variable
The idea is to just add the two integer variables and then subtract each piece out in turn.
Say you have x=5 and y=9.
If you are using Python you can just stop right here as "tuple unpacking" will so all the work for you.
x, y = (y, x)
and you are done. x is 9 and y is 5.
Let's assume you want to do it the harder way...
Replace x with the sum of x and y.
x = x + y
x = 5 + 9 = 14
Now you have "lost" x, but you still have y and you can deduce what your old x is.
For the new y take your total and subtract out y, which will leave you with the "old" x in y's place.
y = x - y
y = 14 - 9 = 5
Now to get the new x, take the total and subtract out the new y. which will give you the old y which goes in x's place. It's more confusing to write it out in English that to just see the math.
x = x - y
x = 14 - 5 = 9
x = x + y [x changes]
y = x - y [y changes: Total minus original y]
x = x - y [x changes again]