A box contains 3 blue, 4 white, and 5 red marbles. If a marble is drawn at random from the box, what is the probability that it will be white?

The correct answer is $\frac{1}{3}$

As the marble is drawn at random, all the marbles have equal chance of getting drawn.
∴ Number of possible outcomes = 3 + 4 + 5 = 12
Let E denote the outcome where the marble is white.
∴ Number of outcomes favourable to event E = 4
∴ Probability of event E = $\frac{4}{12}$ = $\frac{1}{3}$