All Odds Before Even
题目
You roll a fair six-sided die again and again. What is the probability that all three odd numbers (, , ) have appeared before any even number (, , ) appears?
解析
先自己试一试,卡住了再打开提示。
提示 1
Repeated rolls don't matter. Only the order in which each face shows up for the first time matters.
提示 2
That order of first appearances is a random arrangement of , with every arrangement equally likely.
查看解析
Ignore the repeats. Rolling a face you have already seen changes nothing, so we only care about the order in which the six faces show up for the first time. That order is an arrangement of , and by symmetry every one of the arrangements is equally likely.
What we need. All odd faces before any even face means the first three faces to appear are in some order, and the last three are in some order.
Count. The odd faces can be ordered in ways and the even faces in ways, so there are good arrangements:
Another way to see it: the three odd faces have to take the first three of six places. Choosing which of the places are odd can be done in equally likely ways, and only one of them is "the first three". So .
Answer: .