Question 1210613
<pre>
             Marek             Zaven          TOTAL

Start        900-z               z             900

AFTER
SPENT       900-z-0.75x         z-x=0.5x

            900-z-0.75x=(0.75)(900-z-0.75x+0.5x)
</pre>
That does not go directly to the quanity asked in the question. 

System is  {{{system(z-x=0.5x,900-z-0.75x=0.75(900-z-0.25x))}}}
But a more efficient setup may be possible.


(*WARNING:  possible fault in setup; algebraic steps produced faulty outcome)