Solution:

Total number of balls in the bag = 15.

Let the number of white balls be x.

Then number of red balls = 15-x.

The probability of drawing a white ball = x/15.

Probability of drawing a red ball = (15-x)/15

Given probability of drawing a red ball is twice that of a white ball.

(15-x)/15 = 2×(x/15)

15-x = 2x

3x = 15

x = 15/3 = 5

Hence the number of white balls is 5.