How many red marbles?

$\dfrac{4}{n+7} = \dfrac{1}{3} \implies \boxed{n = 5}$

Using $b = 4$ , where $b$ is the amount of blue marbles:

$\frac{1}{3}$ $=$ $\frac{4}{12}$

So the total amount of marbles is $12$ .

$7 + n = 12$

$n = 5$

So, the amount of red marbles is $\fbox 5$ .

The probability of getting a blue marble = $\frac{4}{n+7}$ so $\frac{4}{n+7}$ = $\frac{1}{3}$ hence, $n$ = 5