Solution:

Number of red balls = 6

Let number of blue balls be x.

Total number of balls = 6+x

Probability of drawing a red ball = 6/(6+x)

Probability of drawing a blue ball = x/(6+x)

Given the probability of drawing a blue ball is twice that of a red ball.

x/(6+x) = 2× 6/(6+x)

x = 12.

So total number of balls = x+6 = 12+6 = 18.

Hence the total number of balls in the bag is 18.