x = volume of 10% solution to use in the mixture
y = volume of 25% solution to use
She wants a 200 mL solution, so x + y = 200.
The first solution contributes 0.10x mL of alcohol, while the second contributes 0.25y mL. Together, they make a solution with 0.20(x + y) = 0.20(200) = 40 mL of alcohol.
[tex]\begin{cases}x+y=200\\0.10x+0.25y=50\end{cases}\implies x=\dfrac{200}3,y=\dfrac{400}3[/tex]
