[tex]\bf \begin{array}{lccclll}
&amount&concentration&
\begin{array}{llll}
concentration\\
amount
\end{array}\\
&-----&-------&-------\\
\textit{current sol'n}&4&0.25&1.00\\
\textit{pure sol'n}&x&1.00&1.00x\\
-----&-----&-------&-------\\
mixture&y&0.5&0.5y
\end{array}[/tex]
notice, again, we use the decimal format of the percentage, 25% is just 25/100 or 0.25 and so on
so.. hmmm the quantities, of 4 + x, must add up to y, thus
4 + x = y
and their concentration amount, must add up as well
1.00 + x = 0.5y
thus [tex]\bf \begin{cases}
4+x=\boxed{y}\\
1+x=0.5y\\
----------\\
1+x=0.5\left( \boxed{4+x} \right)
\end{cases}[/tex]
solve for "x"
