The tree diagram for two tosses of a fair coin helps to relate the four possible outcomes to the number of heads for each outcome:
The key idea is to determine how many outcomes (paths through the tree) contain zero heads, one head, or two heads, respectively.
Some people have great difficulty with the notion that some values occur more often than others. They think that if there are three possible values (zero, one, and two), then each must be equally likely. The tree diagram may clarify the different frequencies of occurrence for different possible outcomes.
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