Notice that the box has a total os 4 + 5 + 3 = 12 balls, since there are 4 that are yellow, then, the probability to randomly obtaining a yellow ball in a single draw is:
[tex]P(Yellow)=\frac{\text{ number of yellow balls}}{\text{total number of balls}}=\frac{4}{12}=0.33[/tex]therefore, the probability is 0.33 = 33%
