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.