Find the probability of getting exactly two “head” when flipping three coins.
| HHH | THH |
| HHT | THT |
| HTH | TTH |
| HTT | TTT |
$$
p(2’H’) = \frac{3}{8}
$$
计算骰子的概率
两个骰子一次投掷得到的数值和：
方法1：枚举
| | 1 | 2 | 3 | 4 | 5 | 6 |
| —- | —- | —- | —- | —- | —- | —- |
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 6 | 7 | 8 | 9 | 10 | 11 | 12 |
P(7) = 6/36
方法2：
P(5) = 4 / 36
| D1 | D2 |
| —- | —- |
| 1 | 4 |
| 2 | 3 |
| 3 | 2 |
| 4 | 1 |
