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12-131.

Three coins are flipped. Let $X = \text{the number of heads}$.

1. List all of the possible heads/tails combinations of outcomes.

HHH, HHT, ...
There are eight possible combinations of outcomes.

2. Create a probability model for this situation.

 $X$ $0$ $1$ $2$ $3$ $P(X)$
3. Calculate the expected value of $X$.

$E(x)=\displaystyle \sum _ { } ^ { }X \cdot P(X)$