# White Balls, Black Balls

There are two bags $A$ and $B$. Bag $A$ contains $n$ white balls and $2$ black balls while bag $B$ contains $2$ white balls and $n$ black balls . One of the two bags is selected at random and two balls are drawn from it without replacement . We know that if both the balls drawn are white balls, the probability of bag $A$ being selected to draw the balls is $\frac{6}{7}$. Find the value of $n$.

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