Difference between revisions of "Expected value"
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*Find the expected value of a dice roll. | *Find the expected value of a dice roll. | ||
**Find the expected value of a weighted dice roll, where each dot has equal probability of being on top. | **Find the expected value of a weighted dice roll, where each dot has equal probability of being on top. | ||
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===Intermediate=== | ===Intermediate=== | ||
*In his spare time, Richard Rusczyk shuffles a standard deck of 52 playing cards. He then turns the cards up one by one from the top of the deck until the third ace appears. If the expected (average) number of cards Richard will turn up is <math>m/n,</math> where <math>m</math> and <math>n</math> are relatively prime positive integers, find <math>m+n.</math> | *In his spare time, Richard Rusczyk shuffles a standard deck of 52 playing cards. He then turns the cards up one by one from the top of the deck until the third ace appears. If the expected (average) number of cards Richard will turn up is <math>m/n,</math> where <math>m</math> and <math>n</math> are relatively prime positive integers, find <math>m+n.</math> | ||
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===Olympiad=== | ===Olympiad=== | ||
*{{problems}} | *{{problems}} | ||
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Revision as of 20:35, 19 January 2008
| This is an AoPSWiki Word of the Week for Jan 17-23 |
Given an event with a variety of different possible outcomes, the expected value is what one should expect to be the average outcome if the event were to be repeated many times. Note that this is not the same as the "most likely outcome."
For example, flipping a fair coin has two possible outcomes, heads (denoted here by 
) or tails (
). If we flip a fair coin repeatedly, we expect that we will get about the same number of heads as tails, or half as many as the total number of flips. Thus, the average outcome is 
. Note that not only is this not the most likely outcome, it is not even a possible outcome for a single flip.
More formally, we can define expected value as follows: if we have an event 
whose outcomes have a discrete probability distribution, the expected value 
where the sum is over all outcomes 
and 
is the probability of that particular outcome. If the event has a continuous probability distribution, then 
.
Uses
- Expected value can be used to, for example, determine the price for playing a probability based carnival game. You first find the expected value per player. Then you can charge a reasonable price so that you gain money, but the price isn't unreasonably high.
Problems
Introductory
- Find the expected value of a dice roll.
- Find the expected value of a weighted dice roll, where each dot has equal probability of being on top.
Intermediate
- In his spare time, Richard Rusczyk shuffles a standard deck of 52 playing cards. He then turns the cards up one by one from the top of the deck until the third ace appears. If the expected (average) number of cards Richard will turn up is
where 
and 
are relatively prime positive integers, find 
where 
and 
are relatively prime positive integers, find 
- An equilateral triangle is tiled with
smaller congruent equilateral triangles such that there are smaller triangles along each of the sides of the original triangle. The case 
is shown here, #3. For each of the small equilateral triangles, we randomly choose a vertex 
of the triangle and draw an arc with that vertex as center connecting the midpoints of the two sides of the small triangle with V as an endpoint. Find, with proof, the expected value of the number of full circles formed, in terms of .
smaller congruent equilateral triangles such that there are 
is shown 
of the triangle and draw an arc with that vertex as center connecting the midpoints of the two sides of the small triangle with V as an endpoint. Find, with proof, the expected value of the number of full circles formed, in terms of - We play a game. The pot starts at <dollar/>0. On every turn, you flip a fair coin. If you flip heads, I add <dollar/>100 to the pot. If you flip tails, I take all of the money out of the pot, and you are assessed a “strike.” You can stop the game before any flip and collect the contents of the pot, but if you get 3 strikes, the game is over and you win nothing. Find, with proof, the expected value of your winnings if you follow an optimal strategy.
Olympiad
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