A bag contains 3 red and some blue balls?

The number of blue balls in the bag is 12We will call the amount of blue balls "n"The total of balls is : 3 + nThe probability of drawing a red ball : P(R) = 3 / (3 + n)The probability of drawing a blue ball : P(A) = n / (3 + n)If the probability of drawing a blue ball is four times that drawing a red ball, we have to : P(A) = 4P(R)⇒ P(R) = P(A) / 4Substituting : P(A) / 4 = 3 / (3 + n)P(A) = 12 / (3 + n)Substituting : 12 / (3 + n) = n / (3 + n)n = 12The number of blue balls in the bag is 12.