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 = 5 + 8 + 4 = 17

                Let  R  denote the event 'marble is red

                      W denote the event 'the marble is white',

                      G  denote the event 'the marble is green'

Step 2: Determine the number of outcomes favourable to the event

                (i)  Number of red marbles = 5

                    The number of outcomes favourable to the  R = 5

                (ii)  Number of white marbles = 8

                      The number of outcomes favourable to the  W = 8

                (ii)  Number of green marbles = 4

                      The number of outcomes favourable to the  G = 4

Step 3: Calculate the probability

             The probability to draw red marble P(R)= \frac{\text{ favourable outcomes}}{\text{Total possible outcomes}} = \frac{5}{17}    

      Similarly,

              The probability to draw white marble P(W) = \frac{8}{17}

              The probability to draw green marble P(G) = \frac{4}{17}

               Note that P(W) + P(G) + P(R) = 1.