I was wondering how I can solve this question as I'm a bit stuck.

Question

$P, Q$, and $R$ are each mixtures of red and white paint. The percentage by volume of red paint in $P$ is $30\%$. The percentage by volume of red paint in $Q$ is $20\%$.

The mixtures $P, Q$, and $R$ are combined in the proportion $12 : 5 : 3 $ respectively.

If the resulting mixture contains $25\%$ by volume of red paint, what percentage by volume of mixture $R$ is red paint?

Answer 1

Call the answer $x$ so $$\frac{12\times 30+5\times 20+3x}{12+5+3}=25.$$Can you solve that?

Answer 2

If the percentage by volume of mixture R is $x$% then in $12+5+3$ units of the resulting mixture we find $(12\times30+5\times20+3\times x)/100$ units of red paint.

This must be $25$% of the mixture resulting in an equation in $x$ that can be solved.