3. What is the probability of an event as rare as four heads?

To determine this probability, first you must acknowledge that obtaining four heads is just as are" as obtaining no heads, as the symmetry of Figure 9.1 testifies. Thus, both sides of the probability distribution are implicated by the language of this question, and, consequently, you must add the separate probabilities.

The probability of an event as rare as four heads is p0heads þ p4heads ¼

:0625 þ :0625 ¼ :125.