x+y and x-y thinking 1

x+y=15
x-y=5
Add the equations to receive 2x=20
x=10
y=5

$x+y=15$

$x-y=5$

Add both equations to obtain

$x+y + x-y=20$

$2x=20$

$\Large\frac{2x}{2}=\frac{20}{2}$

$\boxed{x = 10}$

Substitution

$x-y=5$

$10-y=5$

$y=10-5$

$\boxed{y=5}$

Therefore, $x = 10$ and $y= 5$