so hmmm we'll start off by using the decimal format for the percentage, thus 15% is just 15/100 or 0.15 and 7% is just 7/100 or 0.07 and so on
notice, the concentration of alcohol in the 15% is, well, 0.15 :), so for a quantity of whatever, it'd be 0.15whatever
anyway, for water... water has no alcohol whatsoever, so the concentration is 0%, or 0/100 which is 0.00 or a flat 0
[tex]\bf \begin{array}{lccclll}
&amount&concentration&
\begin{array}{llll}
concentrated\\
amount
\end{array}\\
&-----&-------&-------\\
\textit{15\% sol'n}&14&0.15&14(0.15)\\
water&x&0.00&0x\\
-----&-----&-------&-------\\
\textit{7\% mixture}&y&0.07&0.07y
\end{array}[/tex]
we know that we'll be adding "x" water with the 14mL to get "y", thus 14+x = y
and the concentrated quantities will also add up to -> 14(0.15) = 2.1, thus
2.1 + 0x = 0.07y
[tex]\bf \begin{cases}
14+x=\boxed{y}\\
2.1+0x=0.07y\\
----------\\
2.1=0.07\left( \boxed{14+x} \right)
\end{cases}[/tex]
solve for "x".
