Answer:
[tex]\boxed{\text{400 mL of 9 $\%$ and 600 mL of 36 $\%$ brine}}[/tex]
Explanation:
The mass of salt is constant.
You have two conditions
(a) Mass of salt in 9 % + mass of salt in 36% = mass of salt in 25.2 %
(b) Volume of 9 % + Volume of 36 % = volume of 25.2 %
Let x = volume of 9 %
and y = volume of 36 %
[tex]\begin{array}{rclr}\text{Mass of salt in 9 $\%$ + Mass of salt in 36 $\%$} & = & \text{Mass of salt in 25.2 $\%$}&\text{(a)}\\V_{1} + V_{2} & = & V_{3}&\text{(b)}\\9x + 36y&=& 25.2 \times1&(1)\\x + y & = & 1&(2)\\9x + 9y & = & 9&(3)\\27y & = & 16.2&(4)\\y& = & \dfrac{16.2}{27}&\\\\& = & \textbf{0.60 L}&(5)\\x + 0.60 & = & 1&\\x& = &1-0.60&\\&= & \textbf{0.40 L}&\\\end{array}\\\text{You must mix $\boxed{\textbf{400 mL of 9 $\%$ and 600 mL of 36 $\%$ brine}}$}[/tex]
Check:
[tex]\begin{array}{rcl}0.40 \times 9 + 0.60 \times 36 & = & 1 \times 25.2\\3.6 + 21.6 & = & 25.2\\25.2 & = & 25.2\\\end{array}[/tex]
It checks.
