Molarity Numerical on Mixing Solutions
In addition to the equation for the preservation of the amount of substance you've written, there is one more equation you can use for the system, namely the one reflecting preservation of volumes: $x + y = 3$:

$$
\left\{ 
\begin{align}
x + y &amp;= 3 \\ 
2.5x + 0.4y &amp;= 3 
\end{align}
\right. 
$$

$$x + y = 2.5x + 0.4y$$

$$1.5x = 0.6y$$

$$\frac{x}{y} = \frac{6\cdot 2}{10\cdot 3} = \frac{2}{5}$$

E.g. the answer is $x : y = 1 : 2.5$.