2012 AMC 10A Problem 7

WLOG, let the number of marbles in the bag be $5.$ Since we only care about ratios, we can do this. Then there are $3$ blue marbles and $2$ red marbles. Doubling the red marbles gives us $4$ of them. Then the fraction of red marbles is $$\dfrac{4}{4 + 3} = \dfrac{4}{7}.$$ Thus, C is the correct answer.