A bag contains 5 red balls and some blue balls. If the probability of drawing a blue ball is double that of a red ball, determine the number of blue balls in the bag
Let the number of blue balls be x.
Number of red balls = 5
Total number of balls = x + 5
P (getting a red ball) = 
P (getting a blue ball) = 
It is given that,
2 (
) = 
10 (x + 5) = x2 + 5x
x2 – 5x – 50 = 0
x2 – 10x + 5x – 50 = 0
x (x – 10) + 5 (x – 10) = 0
(x – 10) (x + 5) = 0
x = 10 or x = - 5
However, the number of balls cannot be negative
Hence, number of blue balls = 10
4