X+5y=15
2x-y=8
--------------------
x=15-5y
2x-y=8
---------------------
x=15-5y
2(15-5y)-y=8
-----------------------
x=15-5y
30-10y-y=8
---------------------
x=15-5y
30-11y=8
-----------------------
x=15-5y
-11y=8-30
-------------
x=15-5y
-11y=-22
----------------------
x=15-5y
y=2
------------------------
x=15-5.2
y=2
-----------------
x=15-10
y=2
--------------------
x=5
y=2
---------------------
/x,y/=/5,2/
==============