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.