# Exercise C.4.2

How many times on average must we flip 6 fair coins before we obtain 3 heads and 3 tails?

The $\binom{6}{3}$ ways to have three heads and three tails. Thus:

$$ \Pr\{\text{3H3T}\} = \binom{6}{3} / 2^6 = 5/16 $$

As for the expectation:

$$ \E[\text{3H3T}] = \frac{1}{\Pr\{\text{3H3T}\}} = \frac{16}{5} = 3.2 $$

This can be verified by the following line of Ruby:

```
1_000_000.times.map { 1.upto(2**32).take_while { 6.times.map { rand(2) }.count(1) != 3 }.last.to_i.succ }.inject(:+) / 1_000_000.to_f
```