As you can see, the relative frequency of obtaining exactly 1 head was roughly twice
as great as that of obtaining either 2 heads or 0 heads. It turns out that the mathematical
theory of probability can be used to predict that a result like this will almost always occur.
To see how, call the two coins A and B, and suppose that each is perfectly balanced.
Then each has an equal chance of coming up heads or tails, and when the two are tossed
together, the four outcomes pictured in Figure 9.1.2 are all equally likely.