剩下的35张取到1的概率36张卡片，分别1-9的四种花色(方块，红桃，梅花，黑桃)的卡片。
随机取出9张，其中有四张是1的概率是多少？
解法1
P(all 4 cards is 1 in my hand of 9) = (# of ways in which event can happened)/(the total # of hands)
total # of hands = $\frac{363534333231302928}{987654321}$
the # of hands with 4 1’s = $\frac{11113231302928}{54321}$
P(all 4 cards is 1 in my hand of 9) = $\frac{11113231302928}{54321} \frac{987654321}{3635343332313029*28}$= $\frac{2}{935}$
解法2
| 取到一张1的概率 | 剩下的35张取到1的概率 | 剩下的34张取到1的概率 | 剩下的33张取到1的概率 |
|---|---|---|---|
| $\frac{1}{36}*9$ | $\frac{1}{35}*8$ | $\frac{1}{34}*7$ | $\frac{1}{33}*6$ |
