Explanation:
a probability is always the ratio
desired cases / totally possible cases.
we have here a total of 6 + 8 + 6 = 20 marbles.
to not pull a blue or yellow marble is in this context the same event as pulling a red marble.
so, the desired cases are 6 (red).
which we can get directly from the 6 red marbles, or by counting off the undesired cases : 20 - 8 - 6 = 6.
and the probabilty for not pulling a blue or yellow marble (or simply pulling a red marble) is
6/20 = 3/10 = 0.3