Mary is baking a cake. The recipe calls for 3 cups of sugar 10 cups of flour and 15 cups of salt. She already put in 6 cups of flour. How many more cups of flour does she need to add?
Let's say we have <code>x</code> cups of flour, <code>y</code> cups of sugar and <code>z</code> cups of butter. 
We have <code>4x + 2y + z = 18</code>.
Now we need to find <code>x</code> cups of flour to add. 
<code> 4x + 2y + z = 18
-4x - 4x       //subtract 4x from both sides
-----------
     2y + z = 2
</code>
Now we need to find <code>y</code> cups of sugar to add. 
<code> 2y + z = 2
-2y - 2y       //subtract 2y from both sides
-----------
    z - z       //subtract z from both sides
-----------
          z=0
</code>
Therefore, <code>z</code> should be equal to <code>0</code>, <code>y</code> should be equal to <code>1</code> and <code>x</code> should be equal to <code>1</code>.
