Let there be x blue balls in the bag 

$\therefore$Total number of balls in the bag = (8 + x) and Now, P₁ = Probability of drawing a blue ball = $\frac{\mathrm{x}}{8+ \mathrm{x}}$

and P₂ = Probability of drawing a red ball = $\frac{8}{8 + \mathrm{x}}$

It is given that,

P₁ = 3P₂

⇒ $\frac{\mathrm{x}}{8 + \mathrm{x}}= 3 \times \frac{8}{(8 + \mathrm{x})}$

$\Rightarrow \frac{x}{8 + x}=\frac{24}{8+ x}$

x = 24 Hence there are 24 blue balls in the bag.