Here is an almost identical example:

A coin is tossed three times, and the sequence of heads and/or tails is noted. What is the event that heads comes up at most once.

Solution:

Recall: The event E consists of all outcomes in S which are favorable. To make sense of this, we first need a sample space S. Since we are told that, in the experiment, we note the sequence of heads and tails, the sample space is the set of all possible sequences of 3 heads and/or tails:

The favorable outcomes are those sequences containing at most one head. Thus,

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