Winning at Coins

There are 10 10 cards, labeled from 1 1 to 10 10 , lying face down on a table. You pick a card, and if that card is a 10 10 , you win $ 20 \$20 . If the card is not 10 10 you win $ 0 \$0 . What is the expected amount you would win (in dollars)?


The answer is 2.

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9 solutions

Ahaan Rungta
Nov 24, 2013

There is a 1 10 \frac{1}{10} probability of picking the card labeled 10 10 and winning $ 20 \$ 20 . There is a 9 10 \frac {9}{10} probability of picking any other card and winning nothing. Thus, by the Linearity of Expectation Value, our answer is $ ( 9 10 0 + 1 10 20 ) = $ 2 . \$ \left( \dfrac {9}{10} \cdot 0 + \dfrac {1}{10} \cdot 20 \right) = \$ \boxed {2}.

What you are using here is not linearity of expectation, it is just using the definition of expectation. Linearity of expectation is used when you are taking the expectation of a sum of random variables. You can refer to the expected value technique trainer for a longer explanation of expectation and linearity of expectation.

Lino Demasi - 7 years, 6 months ago

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Oh, oops, sorry about that. And thanks for the link!

Ahaan Rungta - 7 years, 6 months ago

there are only two possible answers 0 $ and 20$ . So how come the concept of probabilty is being used . This is really absurd and futile .

Ashish Shukla - 7 years, 6 months ago

I thought the answer would be which ever amount you had a higher chance of getting...

Nathan Blanco - 7 years, 5 months ago
Andhika Rahardian
Nov 26, 2013

There are 10 10 cards , so the probability to get number 10 10 card is 1 10 \frac {1}{10}

because it just 1 1 card from 10 10 cards

cause the probability to get number 10 10 is 1 10 \frac {1}{10}

so the expected amount is 1 10 × 20 \frac {1}{10} \times 20 dollars = 2 \boxed {2} dollars.

Linus Setiabrata
Nov 24, 2013

On average, if you pick up the cards 10 times, you'll win $20 once. In 10 tries you get $20, so on average you'd win $2 "per try".

Simple way of doing it!

Guilherme Dela Corte - 7 years, 6 months ago
Suyash Gupta
Dec 28, 2013

total sum=20 the ways can be attained=10 so 20/10=2

Please elaborate......... I didn't fully understand

Rohitas Bansal - 7 years, 4 months ago

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see....there is only single way in which sum can be attained 20 and toatl 10 possibilities so 20/10=2.......:)

Suyash Gupta - 7 years, 3 months ago

elaborated it below

Vighnesh Raut - 7 years, 2 months ago

Please elaborate..

Bhavya Budania - 7 years, 4 months ago

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elaborated it below

Vighnesh Raut - 7 years, 2 months ago
Nurul Alam Pavel
Nov 25, 2013

mathematical expectation is the sum of probabilities multiplied by the rate..... here, p(selecting the card labeled 10) is 1/1o therefore, expectation E= 1/10 * 20 = 2.

Frederick Corpuz
Dec 27, 2013

In ten tries you are expected to get the "10"card once, winning you 20 dollars 20 divided by 10 equals 2

Neelesh I
Nov 30, 2013

prob of winning=1/10 and winning amount =20$..we can expect 1/10th of amount which is equal to 2$

(1/10) * 20 = 2

Idan Glickman
Nov 28, 2013

You are expected to win $2. this is because you have a 1/10 chance of winning $20 and a 9/10 chance of winning $0. you add the answer to 1/2 X 20 and 9/10 x 0. the answer to this is 2. so the average winning is $2.

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