A pharmacist needs to combine a $2\%$ solution of a medication with a $25\%$ solution (of the SAME medication) to make $9$ litres of a $3\%$ solution. Use simultaneous linear equations to determine how many litres of each solution she needs to mix.
Usually I would have an attempt and ask if I am on the right track, but my head is all over the place with this question.
Let the initial volumes be $a$ and $b$:
$$\begin{cases} a+b=9\\ 0.02a+0.25b=0.03\times9=0.27 \end{cases}$$
Now you can try solving them.