150,671 views
40 votes
40 votes
The probability of winning a three-person game of rock paper scissors is 1/3. If you just

won the last 2 rounds, what is the probability you will win the next round?

a. 1/27
b. 1/9
c. 2/3
d. 1/3
e. 1/18

User George Kagan
by
2.6k points

2 Answers

5 votes
5 votes
There is nothing wrong with your calculation; each player wins with probability 1/3. The point is that these are not disjoint events, so( wins)+( wins)+( wins)≠( or or wins). P(A wins)+P(B wins)+P(C wins)≠P(A or B or C wins). In fact, the sum of the probabilities is the expected number of players who win. (When there are no multiple winners, so the events are disjoint, this number is always 0 or 1 and so its expectation is the probability someone will win, but this is not true in general.) Here (no-one wins)=3×127+3!27=13 P (no-one wins)=3×127+3!27=13, (2 people win)=3×3×127=13P(2 people win)=3×3×127=13, and (1 person wins)=13P(1 person wins)13, so the expectation is exactly 11, consistent with your answer.

Hope this helped :)))
User Xoppa
by
3.0k points
15 votes
15 votes

d

it's because it is 1 event

User DengSihan
by
2.7k points
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