Solution. We could use the formula above directly, but we will demonstrate what is happening here by using a possibility space diagram that shows all the possible

We could use the formula above directly, but we will demonstrate what is happening here by using a possibility space diagram that shows all the possible outcomes of the event “throwing a cubical die and a tetrahedral die”.

From the diagram we see that the probability of throwing a five on the cubical die is

The probability of throwing a four on the tetrahedral die is

Hence at first it may appear that the probability of throwing a five on the cubical die or a four on the tetrahedral die is However, from the diagram we notice that the occurrence four on the tetrahedral die and five on the cubical die appears in both calculations. Hence we need to subtract this probability. Therefore the probability of throwing a five on the cubical die or a four on the tetrahedral die is