A box contains 3 blue, 2 white, and 4 red marbles. If a marble is drawn at random from the box, what is the probability that it will be (i) white? (ii) blue? (iii) red?

Step 1: Find the possible out comes of an event
NOTE: Saying that a marble is drawn at random means all the marbles are equally likely to be drawn.
Therefore, the number of possible outcomes = 3 +2 + 4 = 9
Let W denote the event 'the marble is white',
B denote the event 'the marble is blue' and
R denote the event 'marble is red
Step 2: Determine the number of outcomes favourable to the event
(i) Number of white marbles = 2
The number of outcomes favourable to the W = 2
(ii) Number of blue marbles = 3
The number of outcomes favourable to the B = 3
(iii) Number of red marbles = 4
The number of outcomes favourable to the R = 4
Step 3: Calculate the probability
The probability to draw white marble P(W)= \frac{favourable\ outcomes}{Total\ possible\ outcomes}=\frac{2}{9}
Similarly,
The probability to draw blue marble P(B) = \frac{3}{9} = \frac{1}{3}
The probability to draw red marble P(R) = \frac{4}{9}
Note that P(W) + P(B) + P(R) = 1.